Depending on the platform that the next Smash is released on, wouldn't it actually be easier to transition everything? I mean, we did lose a few things since Smash4, but then you also have to consider that, aside from the new characters and previously cut veterans, they were also giving SEVERAL stages an HD facelift (close to every one across the series' history, including those from the 3DS version of the previous entry), not to mention Omega and Battlefield variants for all of them. In comparison, the number of stages that could be ported directly from WiiU were fairly low (not sure why stages like Orbital Gate Assault and Woolly World couldn't make it, though). Basically, they've already put in a bulk of work that seems like it could be future-proofed, not to mention the heavy lifting for characters like Steve whom, should he return, the base game could already be designed with as a consideration.
And aside from the ATs that were upgraded to Fighters, I can't really say why we lost the ones that we did. However, Ultimate did introduce almost 30 new ones in their place (compared to 9 lost since Smash4, with 2 now being playable). As for items, literally the only ones from Smash4 that were lost were the Spring and the Team Healer (the latter of which may as well have had its functionality replaced by the Healing Sprout and Healing Field). Again, not sure why the Spring was removed, but there are 13 new items in its place. Also, not a single Pokeball summon got cut between Smash4 and Ultimate, with the latter adding on 15 more.
Depends. The only reason we even got these few HD facelifts and new ATs is because the bulk of stages, items and ATs came from elsewhere. I was specifically talking about
new content added for Ultimate.
For Pokéballs: Sm4sh added 23 new Pokémon. Ultimate added 14 - 9 less than its predecessor.
For ATs, Ultimate did indeed add more than 4 (5 more, to be precise). But it also cut more ATs than 4 did in relation to Brawl.
As far as modes go, we lost Coin Battle, Smash Run, Smash Tour, Trophy Rush, Special Orders, Target Smash, Event Matches, several options for Multi-Man Smash and a dedicated All-Star Mode.
In return, we got WoL, Spirit Board, Squad Strike, Smashdown, Stage Morph... and that's it. Keep in mind, the first two, along with the very introduction of Spirits, were very clearly a move to save resources. Not to mention Stage Builder and Home-Run Contest could only come back as DLC.
Gameplay-wise, we only got a few touch-ups on existing mechanics. Even Parries are an evolution of another - Perfect Shielding.
That's what I'm talking about. I don't think what we got is
bad, just that at some point, having to bring everything back means something else has to go - whether it means cutting a few stages, modes and items - or not adding as much new content as previous games did.