The Fool (spoilers for all RotE books, including LST)

[Garetha]

[But who will the next catalyst be, now that Fitz has retired?
[/quote]

I'm not sure he can retire as a Catalyst. The Pale Woman is shocked when she realises he is still the Catalyst, and bringing the Fool back to life, altering the future, is very strong evidence of this. Perhaps Fitz remains as such so that he can play an important role in future books, or maybe I'm just wishing.