Beck being in Tron 3 and having as big of role as you propose there would need copious amounts of exposition and fanfic explanation since he wasn't mentioned or introduced in T:L, and hardly anybody watching Tron 3 will have seen Tron: Uprising. They already did a lot of exposition to ramp us up on Clu, Quorra/ISOs and Rinzler, etc. in T:L... and I'm not personally a fan of exposition.
If anyone survived T:L except for Rinzler/Tron and the digital construct of Flynn, it will definitely need some explainin'. Zuse or Gem 'might' somehow work... but probably/hopefully not. So to throw Beck of all people into the mix of survivors from T:L?? Yikes even more explainin'. No thanks. They'd have to say he was jammin' with Daft Punk at the End of Line Club or something else completely fabricated from thin air.
I prefer to keep it simple (using information we have from the movies) and simply say the Sea of Simulation has made Rinzer be Tron... just as we already partially witnessed on the big screen in T:L. Beck isn't needed to bring Tron back; the Sea of Simulation with it's special properties has already either done that, or got him very close. And I have a feeling if Tron isn't yet 100% through T:L's Sea of Simulation, he will be made so by Quorra (who we've seen has special regenerative capabilities) -- or by Flynn (after he's brought back in either digital or human form), or by Alan who coded him in the first place.
They can make Tron 3's story as complex and cumbersome as they want through inter-weaving characters and events from the comicbooks, cartoons and video games. But many people are not privy to that information and it's not my preferred path of action. Otherwise, here comes more exposition.
I guess I don't mean that Tron 3 should be based strictly and entirely on what we've seen in the movies and they should have no room to expand or be creative, but they should be very careful about giving prominent attention to characters or events we haven't yet seen in the movies.
I'd prefer Tron:Uprising to exist on its own and its events conclude before Tron:Legacy so Tron 3 isn't creating questions from people like, "Who is this Beck guy? Was he in Tron or Tron: Legacy? Why does he have Tron's suit?"
We don't yet even fully understand what ISOs can do and what all the existing Sea of Simulation is capable of -- e.g., giving rise to the existing ISOs, changing Rinzler into Tron -- and you already want to start making another race of ISOs by injecting 'Tron DNA + Quorra DNA" into the Sea? Again, to me this is just overcomplicating things. Let's work with the foundation we have so far in the movies before we start going down weird paths, imo. If ISOs run their course by the end of Tron 3 and we're bored with them and their capabilities, then let's start tinkering with them and mixing DNAs, I suppose, if we really have to. But not in Tron 3... heavens no. Have you gone mad, ShadowDragon1?
