i'm gonna go a bit against the flow here...
first off, i agree that the Nexus behind the Taker Attack is a terrible idea. it promotes the wrong guys on the wrong show. and part of what does make Nexus so awesome is that we have to wait to see them every week. we're not seeing them 3 times a week and dominating every show, a la NWO, but we're seeing them just once a week and, while dominating, still getting beat up once in a while which lends to the credibility and realism of the angle. Nexus doesn't need the Taker story to be over. they're already over. so leave it alone and presume that it's a Smackdown star responsible for Taker.
that said, an obvious choice could go to Kane. it's a proven story that works, the two work okay together in the ring and the story writes itself with Kane wanting the title that his brother was also in contention for. but i think that there are a lot of other possibilities here also...
Teddy Long: his way of getting back at McMahon? take out McMahon's top draw on the show. this could start a heel turn for Long and the start of a new stable.
CM Punk: he has history with Taker including some title switches. so Punk is out for revenge. this might be okay, if written well and executed well.
Rey: this is super risky, but this could start a heel turn for Rey. it's not been done, to my knowledge, ever, and Rey is super over with all the merchendise and kids, but still, maybe his gimmick could use a bit of a freshen up. have him say that he took Taker out so that he could get his shot at the title. don't just make him the Ultimate Underdog. make him the guy that will do sneak attacks to win and to stack the deck more in his favor. if WWE is considering turning Cena heel, they might also consider a heel Rey.
Big Show: only room for one monster kind of deal. only problem here is that Show and Taker has been done as much as Kane and Taker. so this is not such a good option.
Swagger: felt that Taker was the biggest threat to his title. so Swagger regains the title in time for a Taker return and these two can then feud.
Christian, Kofi, Drew, Matt, Ziggler, etc: any mid-card guy that felt this was the best shot to "make a name" for themselves and get a shot at the title. take out the big threat and enter MITB with hopes to win. this would take lots of good writing and execution but could do wonders for one of these guys if this was the case. maybe a heel turn for Christian or Koki and good feud with Taker would ultimately propel them to eternal main event status.
those are some of my thoughts, or at least, some of my hopes for how this plays out.