TTYD's visual identity looks mostly intact, which is the most important thing since it would be the hardest for fans to fix. It's pretty funny how simply removing new Paper Mario's white outline is enough to send a huge chunk of fanboys into a collective sigh of relief. I'm curious to see if they change anything like the progression of some of the chapters like chapter 4, since it's kind of popular in the Mario RPG Brother Wars meta to make fun of TTYD for having a lot of backtracking (which is bullshit.) Or like, a Paper Luigi side thing.
I foresee some pretty heavy localization, though. TTYD has some fairly saucy stuff in it for a mario RPG, and I just don't see things like the peach shower scene or the playboy bunny boos making the final cut. Every Mario RPG past Paper Mario 64 seems to already have very heavy localizations from what I've played, ESPECIALLY Super, but it's not really something that gets a lot of discussion. It probably won't get that much discussion this time either because it's much more important that we have 100 more years of FUCKING VIVIAN DISCOURSE.
They can do whatever they want with it, honestly. The original holds up perfectly fine and it isn't hard to download dolphin, an iso, and open the iso in dolphin. Downloading an HD texture pack and dragging and dropping it into the right folder is also an option if you're a real pro gamer giga hacker.