Well, I already play a potent MK, along with Falco.
Other characters I have that are at performance level that I don't use in tourney are Snake, Kirby, Mario, Zelda/Sheik, Ike, and Jigglypuff.
My Marth is my best character, but recently, I've just been LOSING. Sometimes, I'm worse than my opponent, but sometimes, I just lose to better characters. It's frustrating.
I've been using Falco for MK, but MKs who are good at the MU are winning. I don't think Snake is that good, and if I had to pour more effort into Snake or MK, it would be MK. I find it fun to punish stupidity with Snake, but I prefer MKs options. He's actually a fun character.
I love Marth to death, but he's not really that viable. He has huge weaknesses that only the top Marth's know, and it's only a matter of time before more players start exploiting them (My brother ***** my Marth sometimes with tactics NO ONE ELSE EVER USES)
I'm never going to main ICs, Snake doesn't have what I like. I may pick up Diddy Kong, but I already play and enjoy MK, so I don't see why I shouldn't put him to good use.