well first off lets go with logistics. the original ultimate custom night was almost to big for clickteam, to the point where scott had to cut corners just to fit the game. so in order to make UCN 2 better than the original (or at least bigger) it would likely need to be made on something else. what I think would be best is if it was made by steel wool in 3D, if they can make a game as big in scale as security breach a UCN sequel in 3D would be in the park for them. now the question is, what would it be like? would it be like an extension of the first? having the same mechanics as the original but with more characters/features? would it be completely new, like with a whole new roster and mechanics? or would it take the collective apporach and use the most important charatcers from across the series from fnaf 1 through SB. if we assume it's a whole new roster what would it be? well if we assume it would be another 50 here is what I think the list would be

first it would use the main characters that were not present in the original

golden freddy, endo-01, withered freddy, withered foxy, shadow freddy, endo-02, phantom foxy, phantom chica, phantom marionette, nightmare foxy, jack-o-bonnie, funtime freddy, bidybab, electrobab, yenndo, and candy cadet

then it would start incorporating the games that came after the original UCN such as

glitchtrap, plushbaby, grim foxy, and dreadbear

it's unclear weather or not fnaf AR counts as a part of the main cannon, so like fnaf world I think it would only get a minimal amount of representation

just freddy frostbear and 8-bit baby

then of course we have security breach and it's cast of characters

glamrock freddy, glamrock chica, monty, roxy, vanessa, vanny, DJ music man, toy music man, the S.T.A.F.F robots, the glamrock endoskeletons, burntrap, and the blob

I also think that the secret bosses from fnaf world, freddy in space 2, and fury's rage would be encorporated. the reason being that the only fnaf world character in the original roster was old man consequences, who was kind of a secret character

animdude, chipper's revenge, chica's magic rainbow, scott cotton, the poopet master, the mad theorists, animdude's revenge, mechiplier, and the dreamgeist

it's possible that the deedee roster from the original would be playable

shadow bonnie, nightmare chica, plushtrap, bonnet, minireena, and lolbit

the last 4 I think would be secret characters that would have to be unlocked and would have major lore signifigance.

those being fredbear, springbonnie, princess (from princess quest), and maybe even afton himself.

however what I think is the most plausible is that, like the first, the game would be a celebration of fnaf in general, and if made by steel wool could be made bigger than the first and would have a roster consiting of the most important characters in the main series. I think this roster of 75 makes the most sense, I made sure it covered all of the mainseries games from fnaf 1 through security breach (did not include fnaf Ar because it's questionable weather that is cannon, if it is I'd edit it to include 8-bit bbay and frostbear)

freddy

bonnie

chica

foxy

golden freddy

toy freddy

toy bonnie

toy chica

mangle

BB

withered freddy

withered bonnie

withered chica

withered foxy

withered golden freddy

the marionette

shadow freddy

shadow bonnie

springtrap

phantom freddy

phantom chica

phantom foxy

phantom mangle

phantom BB

phantom marionette

nightmare freddy

nightmare bonnie

nightmare chica

nightmare foxy

nightmare fredbear

nightmare

plushtrap

jack-o-chica

jack-o-bonnie

nightmare BB

nightmare mangle

nightmarionne

baby

ballora

funtime freddy

funtime foxy

ennard

bidybab

electrobab

minireena

bonnet

lolbit

rockstar freddy

rockstar bonnie

rockstar chica

rockstar foxy

happy frog

mr hippo

pigpatch

ned bear

orville elephant

music man

funtime chica

el chip

molten freddy

scraptrap

scrap baby

lefty

glitchtrap

plushbaby

grim foxy

dreadbear

glamrock freddy

glamrock chica

Roxy

Monty

the daycare attendant

vanny

DJ music man

burntrap

now of course there is also the possibility that it could be expanded upon, I think a cool idea would be a concept of DLC which could add new characters to play with, perhaps even digging into the non cannon games, or even the fanverse if they were bold enough.

UCN is my favorite fnaf game out of the lot, but I think a sequel made by steel wool could be stellar, what do you think would be made of a UCN sequel?