And who is to decide what is "interesting"? I could say Mewtwo was stall as balls with his game revolving around getting a throw in.
When a character plays in a way in which they can utilize a lot of options, especially if they have a general different feel than the other characters, I'd say that most people would say that is interesting. Now sometimes a problem could be that a character is so weird that it breaks the feel of the game, but that argument doesn't appeal to me as stated.
Probably one of the reasons Mewtwo (not a competitive Melee player here) would have been so one-sided gameplay-wise is because his other options were flawed in some way. So now that the power is in the hands of the P:M developers, I think that Mewtwo could have his game brought to what it should have been. Mewtwo was a unique character that pretty much stayed in the game, and I think that now people know what we could do to him, especially with brainstorming.
So if a clone-engine isn't possible, I don't think that the team should rule this out, especially in light of the target audience. I wouldn't like it if I was a Lucario player either, honestly.
I like mewtwo as a pokemon, but I'm not going to beat around the bush. He was terrible. I honestly don't think over half the melee community even cared that he was in melee and only really gave a **** after the fact that he was gone.
People didn't pick him because he sucked. I know I felt that I wish I could have played him, but he was just that flawed. So, I don't think that a functional Mewtwo would be less appreciated, because honestly I think a lot of people liked him but knew/experienced that it wasn't going to work. If you think of his moveset and what could be done to tweak it, I think you might be able to see what I'm saying in comparing the two.
I don't know that it would do any good, but I think if we got a discussion on the flaws of Mewtwo, his moveset, what could be done vs. the gameplay and the like of Lucario that people would see if it was more worth delving into. The thing is that Lucario might do well to have some of those options and the ideas of Mewtwo, but if the clone engine is possible in the future, obviously things might be better separate. At any rate, I think a merger would be an appealing middle ground.