Honestly, I want them to keep as much stuff "original" as possible. Hopefully they will fix the overall gameplay and AI and make it where they can correct any future exploits as needed (DLC).
In regards to visuals, obviously the new actors shot in HD will be huge. I'm hoping the backgrounds look similar, if not better, to the Palace Gates fake that was posted. I know some people would like the Fatalities and such to look more polished, but I've always been a fan of the old school style (10 hands, 4 rib cages and 3 skulls, lol!). I understand the desire to make it seem more real, but to me, that would take away the nostalgia factor. That being said, if they changed it, I wouldn't be devastated, maybe just a tad bit disappointed.
Regarding unlockable bosses or playable hidden characters, I vote "No." I know lots of people would disagree, mainly because it was their dream to unlock Reptile in MK1, find the error macro himself, Ermac or even play as Noob, Smoke or Jade in MKII. The fact is, we never got it then, so would it really be that big of deal now? I like the old days when there were aspects of games that were dreams and dreams only. I say don't fix what isn't broken. In this case, leave the NPCs non-playable.
Multiplayer? If you mean 1-on-1 VS... of course. Online? Yes. Tag team? Oh hell no! That should be in MK9 and any future games, don't mess with the classics, though.
Here' something that you didn't mention specifically, and would seem to go against what I've been saying, but bare with me. I think the game should include HD remakes, separately, of MK1, MK2 and UMK3. But I think there should also be a "Trilogy" styled HD Remix game. It should include all the characters and arenas from the other games, but put them into this one mode (that includes the different versions of the characters as well .... i.e. MK2 Jax -and- MK3 Jax, both unique). Finally, and possibly most intriguing, would be to include a few "new" characters. By new I mean characters that appeared in the 3-D era of MK games. Can you imagine how cool it would be to play as Quan Chi in the classic, 2D visual style? We got a glimpse of him in Mythologies that way. I wouldn't go Armageddon in this Remix mode, but add some major characters from the 3D era (say the top 4 or 5, perhaps). If they're refilming all the old sprites, what's a couple extra ones, right?