The Colossus probably would've been used to maintain peace in the old NTF systems for some time, assuming it survived Capella. After that, maybe using it to spearhead a mission to Earth (which seems to be the general theme for post-capella campaigns). At the least it could be used for a command ship during any fleet operations.
I can't see the GTVA building more Colossus scale or larger ships after Capella, at least not without some very good reason to do so. It took them about 20 years to build it in the first place. Did the NTF Rebellion play in fast-tracking its completion? And how much wasn't finished because of it?.
Even given that doing it a second time might not take quite so long, I still can't see it taking less than 10 years to build another hull. They'd be better off just building smaller destroyer or corvette sized ships with powerful weapons systems, which individually could be used to meet multiple threats but could also be pulled together for a more powerful fleet offensive.