Going from goatalope's specs the current chain capacity on the CAAD is 50-34 + 26-12 = 30 so with a long cage rear d a cog with a range as wide as 12-32 could be used with the current chain rings and not exceed the derailleur cog limit or capacity.

That would give a nearly 1:1 low gear or 28 gear inches which should handle some exceptionally steep grades and be good for everything except loaded touring where the lows are usually lower.