I decided to take a look at Capcom's franchises and unless I missed someone there's only one Capcom protagonist who isn't able to fight. Would this normally be an issue?.
Villager... and he is in MVC3...
Don't do this to you. Phoenix biggest hurdles are just Dante, Monster Hunter, Morrigan and Amateratsu... and maybe Resident Evil?
Still, there is tons that could be done with Smash Ultimate DX withouth putting too much ephasis on newcomers and asuming that it's gonna be on a console with more processing power.
New costumes for every character could be an easy selling point to use it can be a new costume based on either teh character's history or a costume that makes sense with the character the character with 4 recolors (Twilight princess zelda, wedding bowser, Bayoneta's 3 bayonetta, Light suit Samus,) or if the character doesn't have very interesting alternatives like Captain Falcon, create one (like sharp dressing captain Falcon, etc..) Plus one extra "fun costume" as a character cosplaying as someone from a spirit battle (Kirby as a DR mario virus, Zelda as Charlotte Aulin, Donkey Kong as Goron, Etc..) Also would help to give teh game a fresh look.
This would make it tempting for people that want to play the game and would otherwise watch let'splay on the singleplayer content.
More Bosses would be certainly fun little challenges that don't alienate teh balance of the game, plus you can rep even more characters (Eggman, Giratina, Kraid, etc...)
More modes specially to spice 8 player smash. and big stages. Coin battles could create interesting team dinamics in say 4v4 scenarios. And a king of teh hill mode (or board the platform) could really spice a lot big stages like palutena temple.
While charcter based break the targets certainly are a big ordeal considering the amount of characters, one per character is still pretty doable considering you have resources for a new game and you are reusing teh meat of teh previous game, meaning that you don't have to develop much stages and stuff)
Assist trophies could also get a little overhaul, mostly in a game mode where you could choose to fill a bar or fulfill some criteria to summon them, rather than random spawning, in order to implement them into battle strategies. and maybe some could do assist attacks kinda like how assit worked on teh first Marvel vs Capcom game.
Smash run could also make a comeback, with enemies and all.
Classic mode is actually quite cool but still could get a big makeover. more bosses, modes and characters (the DLC) means that you could make a very fun minigame in itself Add the personalized break teh targets as a bonus stage, and add different layers to the other bonus stage (like designing 6 more tracks, and designate one depending of teh difficulty) and add some silly ministory or flavour text to teh penultimate battle (wouldnt be as complicated as creating one palutena guidance for every character, big but doable) and implementing the other bosses, and better implementing Assist trophies and even smash run enemies into the "opponents roster".
Spirits could also recieve an overhaul, writing a brief flavour description for all of them would also be looong, but we are considering that you are using resources that don't need to be destinated to recreate the entire roster since you are reusing it.
With all teh new resources and modes, i think it would be pretty doable to do an interesting World of light 2.0, while not too time consuming, (and is teh kind of content that would help to sell teh game) Also giving the NPCs (trainers, Shop keepers and Explorers) the "Stage cameo makeover" ( a model and a couple of animations) along with some quirky chit chats would also help represent characters that otherwise wouldnt be represented in a economic way. (maybe also Reuse assist trophies as NPCs?)
Also, once you unlock a spirit determine a character with a specific spirit setup to use against it and treat it as a "locked spirit fight" that would completely replace what were events in other smashs, and that you can access whenever you like in the Spirits gallery. you stil lwould be able to fight said spirit however you like in world of light and spirit board, but the locked one should be more challenging, annd a way to flesh more teh spirits battle, instead of teh normal battles than usually ended being a little too inconsequential. Between this and teh flavour text, i think that would make teh spirits waaay more interesting as a gallery, and more exciting once they get added like in events.
At this point teh game could have such an extent of character models, stage cameos, stages, bosses, enemies, etc etc etc... that an interesting snapshot mode that let put different models and animations in a stage and take pictures and it would be practically a new game mode on itself. and give a ton more worth for people that aren't into competitive experiences.
Also Palutena's guidance and codecs for the newer characters would also be welcome.
AAAnd i'm not even talking about a mii fighters overhaul that could give more value to mii costumes, a stage builder expansion or a exploration metroidvania collectathon that could be created with all this asssets..
Basically, there is a ton you could do with such an amazing library of characters and assets if you could allocate resources for a new game withouth having to remake the roster or heavily expanding it in order to sell it as a new game.