Hoping we don't get any cut characters between smash 4 and 5. i absolutely HATE seeing characters cut out. well.. except for pichu. i don't think i will lose sleep over a pikachu that hurts itself.
as for newcomers... inkling is pretty much there, so that's one highly-requested character out of the way.
personally, I'm predicting a new DK rep, either dixie kong or K. rool. or both, ya never know. maybe a new mario rep (either waluigi, daisy, or toad... toad believe it or not is my most wanted mario newcomer to complete the quartet of mario, luigi, peach, and toad from smb2. would be a great time to introduce melees mushroom kingdom 2 stage)
zelda characters... personally, I'd like to see a newcomer, or heck, re-invent ganondorf to be less like captain falcon.
for kirby reps, i'd want to say bandana dee... but... marx might also be a canidate thanks to star allies. maybe, even as a surprise, galacta knight?
for pokemon... it is GOING to be something from either gen 7 or 8... depending on how close it is when smash 5 launches... probably a starter. hoping a grass starter. as i said before though i'm hoping no cuts are made to pokemon, specially because of the outcry of mewtwo fans (myself included)
and, as per usual with smash, there is GOING to be an nes-era character that comes right the hell out of left field. mr game and watch and ice climbers were in melee, pit and rob were in brawl, and smash 4 gave us duck hunt. smash 5 i'm going to predict the balloon fighter and takamaru.
also ice climbers will return.
as for stages. i'm going to predict returning stages:
64: hyrule castle, peaches castle, kongo jungle, dreamland, and saffron city.
melee: temple, onette, yoshi's island (melee), brinestar, big blue, and corneria.
brawl: port town aero dive, castle siege, lylat cruise, great sea, pokemon stadium 2, battleship halberd, delfino plaza, bridge of eldin, and sky world, pitcochat, mario bros, and 75m.
smash 4 (both systems): duck hunt, colloseum, pryosphere, unova pokemon league, palutenas temple, mario galaxy, pilotwings, great cave offensive, wii fit stage,
all i could think of. probably more, probably less.