I could easily see Diskun being able to move in a similar way to Mr. Game & Watch.
And what would its pose change to? It doesn't have arms. It just has a head, two giant eyes, and two nubs for feet. It can't pull out various random things like G&W does, due to not only not having been in the games that the random junk is a refference to, but also because it has no arms to pull anything out with.
With a new generation, one of Brawl's Pokemon reps is likely to get the axe to make way for a pokemon from the latest generation. Perhaps that was a bad reason for Mewtwo to come back, given that it wouldn't make sense to cut one character for two, but if there IS a four-character limit for SSB4, Jigglypuff and Lucario are the Pokemon reps most likely to get cut.
Also, aside from being cut for Brawl, I don't think Mewtwo's chances are getting worse with time. Being one of the original pokemon, he is a classic and will only become more of one as time goes on.
Sure those two are the most likely in comparison to Pika and PT, but that doesn't mean they would get cut in favor of Mewtwo. If that were the case, we wouldn't be talking about this at all, as Mewtwo would be in.
And yes his chances do decrease over time, because while he may be a classic Pokemon, we already have 5 of those in Brawl, with only 1 from another gen. The next Smash may or may not keep all the Pokemon characters from Brawl, but if it does add in a new character, it will certainly not be a first gen.
And for Kuma Oso's comments, the four character limit, as I said, was a Brawl thing. It doesn't really reflect on the past of future games. I also don't quite think that Pokemon got four because Mario did, because under that circumstance, they could have just thrown Doc back in, and that'd be that, though it still could be a possibility.
In any case, I very much doubt that Mewtwo had a significant, if any role in SSE. If he did, then he should have just gotten in over Jigglypuff, who had no role what so ever. If I had my quess, it'd be that both were not planned at all, and the story was made without them, as at the time, they didn't know who they would pick. Lucario on the other hand was probably decided much longer than either of the two, and that's why it was never about Mewtwo vs Lucario, due to it other wise being completely Gen 1, but rather Mewtwo vs. Jiggs.