To be completely honest, I would absolutely be ok if TR pretty much did ONLY soundtrack work from here on out, with an occasional HTDA record here or there. I'm way down for more NIN obviously but this seems like a natural progression to me,
I think it's incredibly exciting to think about him branching out to work with directors other than Fincher. Tom Tykwer, Gregg Araki, Terry Gilliam, Rian Johnson, Duncan Jones, Aronofsky, J.J Abrams, Quentin Tarantino or even Christopher Nolan are all mindblowing possibilities that would challege TR to go reach new levels of sonic awesomeness. Nevermind thinking about possible colabs with other composers such as Zimmer or even former colleague Clint Mansell, similar to how Zimmer and Howard worked on the TDK soundtrack, etc.
As much as I loved their work on The Social Network score, I thought it was a part of the film that served to make it better and consequently loved the film much more than the score. Nothing against the TGWTD film, but I think long term I'll find I reserve much more respect for this score than its film in this case (there was so much great work not even featured in the film in this case).
So ultimately for me, imagining how TR and AR could grow working with someone else other than Fincher is extremely exciting. Nothing against Fincher though, in fact, if they do end up doing the whole trilogy, I would be super stoked to hear their score for Hornet's Nest, as the nature of that film would lend itself to the exact type of thing TR and Atticus are great at (if you've heard the score for the Swedish version of HN you will likely understand what I mean in terms of its heavy use of glitch and drone).