Probably none of the above.
Leo Komarov would be an option, but due to his lack of "eliteness" the media will pick him apart and expect him to be the best player which he'll never be. He's a great guy, has military background, well-travelled, smart, speaks 5 languages so he can communicate with all the guys in the room and everyone loves him (except opposing teams), but, like I said, the media would expect him to be something he's not and fans would eventually follow the lead. So he's a no-go.
Nylander, let's assume he reaches his "realistic" potential and becomes a 30 goal, 40 assist, playmaking centre on the top line. He's possibly got it if he makes that happen, but I have no clue what kind of leadership this guy actually brings to the table. So currently I wouldn't suggest him as the next C because he's NOT a 70pt player and we have no clue whether this guy could lead water out of a bucket if the instructions were under the bottom.
Rielly, possibly, but again, what is this kid's ability to actually lead? On the ice, sure, in a couple of years, but off it? I haven't heard any rumblings about him being a good locker room guy, someone that speaks up. It's a tough sell. He's probably the best option in terms of being a leader ON the ice, at least from a media POV.
Kadri, from what I've seen, probably not. He's good, but takes the odd shift off, and doesn't seem to have a commanding presence in front of the camera. I don't think he would do well with the captaincy.
Gardiner, Gauthier, Percy, Finn are definite no-gos. Until these guys make the NHL (or in the case of Gardiner, establish consistent play in the NHL) they're not in the conversation.
IMO, it's going to be someone outside the organization that is brought in, who has prior C experience which worked well with the other organization.
I'm going to say, maybe, just maybe, if Olli Jokinen isn't dealt at the deadline and the Leafs deal Phaneuf in the off-season and they then convince Olli to sign another 1-2 year deal, they might make him C as a sort of "interim" captain, while they look for the "next guy".
Guys I would like:
Mike Richards
Keith Yandle (wears 'A' in ARI/PHX)
Ryan Callahan
Michael Cammalleri (wore 'A' for MTL/CGY)
Patrick Sharp (wears 'A' for CHI)
Antoine Vermette (wore 'A' for OTT/PHX/ARI) <- signed as UFA in the off-season