So we're specifically picking a character who is seen as unlikely?
If this is the case, I pick Mike Jones from Startropics.
Sakurai's been putting retro throwback into the games since Melee when he put in Mr. Game & Watch, Ice Climbers, and then later R.O.B., Pit, Duck Hunt...
If he holds to this pattern, it seems likely that Smash 5 will get a new retro character. Most folks seem to think he'll go with Takamaru, and while that one is slightly more likely, I honestly think that Mike is a better choice.
For one, his game was released outside of Japan which is a big get over Takamaru. He's also got a lot more unique stuff to offer. Sure, Ness uses a yo-yo, but I think there's a lot of yo-yo potential that Ness hasn't gone after. In addition, I think that Nintendo isn't wanting to be done with the classic Zelda formula, but at the same time they say that all future Zelda games are going to be in the BotW style.
Back in the day of Startropics' release, it was criticized as being too close to Zelda and not standing sufficiently apart from it. Now would be the time for Nintendo to bring it back if they wanted to keep those kinds of games on the platform but focusing their Zelda team on bigger-world adventures. As Pit shows us, nothing does more for the resurrection of a character and series than Smash Bros representation. Plus, since when has Sakurai made a habit of always being predictable? Note how he got the Inkling reveal out of the way quickly... because we knew they were coming. Hell, he unveiled them before he even unveiled the game itself (kinda). I think this is the kind of curveball that Sakurai would throw, and I think it could be glorious.
Plus I love the idea of just some guy with a regular name being in Smash Bros. "Ganondorf! King Dedede! Palutena! Mike Jones!"