Isaac is my primary choice, mainly because we need more modern Nintendo franchises represented. There are quite a few post-2000 IPs that don't have a character yet.
The problem is, there's not that many noteworthy post-2000 IPs left to include. There's Rhythm Heaven and Splatoon, but that's kind of it as far as feasible characters for Smash are. We already have a lot of the important ones, such as Animal Crossing, Pikmin, and Wii Fit.
I know there's Golden Sun, Advance Wars, Wonderful 101, Starfy, etc but it would appear that Sakurai isn't super huge on giving those types of series playable characters (assist trophies on the other hand...). Whenever there's a second party developed franchise with playable fighters in Smash, it tends to be a huge deal for Nintendo, like Kirby or Pokemon, two of their most popular franchises. Something like Wonderful 101 or Sin & Punishment just doesn't quite compare.
But anyways, I definitely expect (and hope!) for Wolf to be one of the slots. I
really hope the Shovel Knight rumor isn't true, but if it is, I won't be that angry, just kinda disappointed. Same thing with Professor Layton.