I get exactly why people want these characters, I just happen to be on the opposing side that doesn't want non-game characters in Smash.
Regarding things like Marvel vs Capcom though, the name doesn't define the game, the game and the ideas behind it are where the name comes from. Marvel vs Capcom was made specifically to be a crossover between Marvel and Capcom. Similarly, Smash Bros. isn't called Nintendo All-Stars Battle Royale (
), but that's pretty much what it is. Guests are very special cases, which Sakurai has specified as needing to be the best of the best from games. Having third parties in the first place does open the floodgates, but it's something I hope they continue to be very careful with.
Could Sakurai (or his successor as director) change his mind on this in the future? Sure. (I just hope that they don't.)
I'd personally just rather see other crossovers be made to satisfy those. Maybe CN can put some money into actually making a good game to serve as a sequel to that Smash clone they made that one time. Stuff like that.
I do generally agree though, I always viewed guests in MK and SC differently for whatever reason, but I guess it kind of is the same thing. I guess since those games aren't already crossovers and guests were always carefully implemented (well most of the time at least; see Soul Calibur IV) it didn't really bother me as much.
I suppose for me is that I think there's a point where there's too much. With Nintendo's entire library of characters and plenty of third party games as viable choices, I'd be fine if they stuck to that. I just don't want them to lose focus and allow it to essentially become MUGEN.
Third party games is already a massive floodgate. Movies, cartoons, anime/manga, comics, etc. are multiple different beasts altogether imo. And I'm also probably just boring when it comes to my crossover desires.
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tl;dr - I'm boring and just don't want crazy crossovers as much as others unless they're extremely specific like Marvel vs Capcom, etc.