We are trying something different from the usual review / synopsis, and this is an article of wild speculation on the Truman "universe" and what events would take place AFTER the events on screen.
The movie knows to end exactly where it is supposed to. It is a symbolically signific point to end on, because you are cut off from contact with Truman at the same time the in-story audience is. It is also a good place to end it because, what happens next is bound to be inevitably complicated.
What does he (Truman) walk into?
A thought occurred to me. The domed area encompassed is surrounded by water. What that means is: as soon as Truman stepped out of that door, he just fell straight into the Gulf of Mexico. We can presume that since a door was there, there also could have been a little dock there instead of a straight drop into the sea, but it is funnier the other way, and still, that means the very next thing that happens is, he is standing on some little platform in the middle of nowhere that presumably only exists for emergency maintenance and/or is just a disused rotting dock leftover from the dome construction that happened almost 40 years ago. Maybe he swam, this is presuming he is a good swimmer; which the movie establishes he is not, and that he would know which way to swim to get to land and then ... walk until ???? Maybe the dome has a contiguous outer platform around all its shoreline. Still a hell of a walk.
What happens to the infrastructure?
The Seaside Dome was established as a very elaborate and large-scale operation. The dome was visible from space, filled with the latest most technologically advanced camera equipment of the time, had its own self-contained weather control system. It is something too big to simply abandon. Not that it would matter to its owners, but surrounding towns and cities were likely economically dependent on selling housing and amenities to those who worked the dome; the large staff it must take to keep it running such as: various technicians and maintenance staff that would have to do their work under cover as townspeople to not spook Truman, but aren't really actors and wouldn't want to or couldn't stay in character as permanent residents of Seaside. In addition to the vast resources on site is the Truman broadcast, and the further infrastructure to support that, it was also mentioned there was a separate channel that just showed highlights apart from the live stream. The Truman themed bar, which was a part of the official corporate franchise. They wouldn't simply roll it up, but would try to salvage in some way, and there are a few approaches.
1. opened to the public as a tourist attraction
This gives a venue to offload leftover Truman merch and memorabilia as public interest in Truman would be inevitably declining and letting people explore the set would do a little to slow that decline in public interest while they sell off the props. They could gradually introduce some rides and transition into an amusement park; and thanks to the dome and its artificial weather system, it would be a Florida theme park that can't be rained out, which gives them an edge over competition. They could just have it be 79 and breezy everyday with their outdoor air conditioning system.
The dome's (literal) overhead was supported by a 24-hour broadcast that got Superbowl level ratings every day. A ridiculous premise when you think about it, but that is what the movie gives us. They would have to charge admission near the range of what other destination parks are charging to be competitive, but that wouldn't support the power and maintenance on keeping an entire Florida city air-conditioned year-round.
They have to cut corners, and the dome isn't getting any younger. One day, a loose dome tile falls on someone and the litigation from that ends it as a theme park. The dome is dismantled, and the undomed platform gets bought by someone else and becomes a kind of inconvenient regular amusement park on a Sealand platform.
This could work well for a sequel that focuses on Truman struggling to adjust to a normal life. He would be older enough to not be readily recognizable, and wearing a hat and sunglasses, visits the theme park, and gets to see what remains of the place that was once his whole world.
2. Truman comes back and they keep doing the show
Truman only has experience being Truman. Not only is he lacking in real life work/life skills, always being clocked as Truman would make it impossible to settle into a mundane innocuous existence. Besides this, there is nothing saying his core desire would be to just live as a regular guy. That was something, I always assumed, but he probably left the dome with grand expectations. Based on his upbringing and his probable mindset when he left, the early days are a lot of grand stereotypical "best life" activities like sky diving and mounting climbing, and even though not his fault, not exhibiting narcissistic behavior would be an existential challenge for him. He was always the center of attention in Seaside even if not knowing how or why. This is his "normal".
So, after some stumbling in adjusting to living in the general populace, he decides that reality sucks and reaches out to Christoph and begs for his old job back. Of course, it isn't the same, and the ratings are nowhere near what they used to be. At first, Truman is just playing along, but at a point he snaps and makes himself forget it isn't real, and the old status quo resumes, which kind of creeps out everyone. Truman Show has to be cancelled and being forced out of existing in the Seaside reality pushes Truman deeper into insanity. Things end badly.
3. They buy another baby and start over
Nothing is to say they were not allowed to do this. Everything about the dome is already built to serve this purpose. More to this, they now have the experience with the first Truman to improve Seaside and keep New Truman from discovering the truth like his predecessor did. It is also their only shot at maintaining the hype needed to keep the patronage and viewership the operation needs to remain sustainable.
Speaking of "hype", they are going to need a hook to maintain public interest and indicate that it isn't just going to be the same thing all over again. Unfortunately for New Truman, a good twist would be, orchestrating New Truman to have a difficult and challenging life. A life filled with drama and conflict. A product of a lower income single parent household. Dad was a one night stand he/she never gets to meet; mom is addicted to something. School is full of bullies. Maybe Truman is non-white and has to deal with some level of racial prejudice. This is going to be the more intense and grittier Truman reboot. The tactile adversity would also be a distraction from questioning the nature of his/her reality.
It is established that a significant source of Truman show income was through product placement, which creates a new logistical challenge for the show runners. If they start with a new Baby Truman in 1998; after Original Truman escapes, New Truman is going to be starting early adulthood at about the 2020s. This means the biggest sponsors and most profitable product placements are going to be technology. New Truman is going to have to own a smart phone, and possibly a home PC and/or other internet connected devices. To demonstrate these devices being used normally, which is going to need to happen for the product placement, New Truman has to be able to get on the internet. They would have to build a very elaborate fake internet, with, fake You Tube, fake Facebook, fake Porn Hub, fake everything with a fake userbase to ensure that any reference to the existence of the Truman Show isn't seen. In addition to making a fictional clone of at least several dozen popular and large websites, they would need to jam over the air signals in and near the dome so only their own were receivable. Not impossible if you accept the original movie premise, just an aside.
This could be interesting for a sequel where old Jim Carrey as Truman has to play the dad to New Truman and struggles with keeping the secret from New Truman.
What does/can Truman do out in the world?
If you remember, there was this character; Sylvia AKA Lauren, who was a cast member, and befriended him in college in the hopes of revealing the truth to him. Later we see he made a creepy collage that resembles her, that he hid away, and saved the button she gave him. Sylvia turned out to be a member of an organization that protests Truman's captivity and works to get its members hired as extras on Truman Show in the hopes of disrupting the production, which had ultimately not succeeded in forcing the studio to free Truman.
Obviously, Truman would be willing to pursue a romantic relationship with Sylvia, but Sylvia's willingness and intentions are not quite that clear. Does Sylvia like Truman? .. as a person generally? Yes, maybe. It is also possible she sees Truman as a mistreated circus animal from her perspective. At any rate, finding Sylvia is going to be a top priority for free Truman. The problem is: Truman is a wierdo. In no way his fault, but he has no experience in interacting with anyone who isn't an actor role playing. The sad truth would be that Truman isn't coming into the real-world equipped for any kind of real relationship, romantic or otherwise. I think Sylvia would probably give him a shot because their union was mystified and Truman being a very famous person, but unfortunately, Sylvia will be the one he learns by making mistakes with rather than his "true love".
As far as how Truman provides for himself, he probably has to keep being Truman. He would have a book that he signs, and work conventions and sign autographs and talk about the show, the obvious irony is, still being trapped by the show despite escaping the physical set. Sylvia pressures him into joining the anti-Truman show group and advocating for the release of New Truman. Further irony because a lot of Truman's livelihood comes from fans of the show. Yes, logically, Truman should care deeply about the plight of New Truman, but maybe not that much, not even because he is selfish, but because HE isn't actually free yet.