No one's going to (want to) leave unless we keep under-performing in the league and won't make it into CL spots, which is pretty unlikely.
Everyone's on long-term contracts and even the "stars" like Götze/Reus are supposed to have release clauses that activate in 2014.
Only Lewandowski (2014) or Subotic (2014) are more or less likely to leave. Klopp's sort of like a father figure for Subotic so I don't think he'll leave either.
Großkreutz & Schmelzer (both 2014) are very likely to stay as well.
Gündogan (2015) came from a lowly club and signed a contract after Sahin's costly departure. It sort of shocked the club (because he had a very low rel. clause built in) as they didn't seem to pay too much attention to such clauses before, so I really don't think Gündogan has a release clause at all.
Why another DM? I'd say someone like Mainz Schürrle:
Schürrle
Reus-Götze-Kuba
Could be awesome but would change a lot at the same time. We'd need to completely change the way Hummels and Subotic play. The long balls wouldn't work without a true striker and I imagine we would play like shit for some weeks (months?). We should just get that other cheap Polish guy lol.
We have 2 Schürrles: Reus & Perisic.
With second technically strong/play-making DM we don't need long balls, we didn't really need them in 2010/2011 either. This long-ball tactic was sort of born out of necessity because Gündogan couldn't replace Sahin right away. It works, but we don't have to play this way.
So if we had someone like Leitner next to Gündogan, but someone who's also strong defensively, we could sort of a do Spain/Barca tiki taka.
Neither Bender nor Kehl (who won't be an option past 2013 anyways and will probably just sit around like Da Silva in 11/12) can do tiki taka, so when we'll sign a substitute for Kehl I hope he'll be close to someone like Biscuit or Yaya (of course I step below them).
Klopp did already sort of try that in the last cup game (of course with Lewandowski, but also with Leitner/Gündogan).