I don’t know if anything special is going to happen with the last Titan, but would it be worth considering at this point which of these last two would be better kept as the last Titan. Perhaps in case Thargoids become a power centered around it, or some other reason?
I don't think it particularly matters. Aside from naming by humans and the sigils painted on each Titan, they are largely identical, and differences only arise from local positioning influencing possibilities of attack, and where spires are placed (as well as how many). Inner core density of Titan control spheres also used to matter more but since they rebalanced things...
Of course, Raijin is a better name than Cocijo (in my opinion) but after the work which has already gone into clearing out the former's control systems, it's probably going to be Cocijo which is the last one to go. The Powerplay Q&A thingy published in text form today seems to suggest that they will be around in some way following the update anyway...
... does Cocijo impede on any Powerplay spheres, anyway? I know Raijin does with proximity of Utopia and some of LYR/Sirius, but that's as far as my knowledge goes. And depending on the pace of clearance Raijin's interference will either be neutralized completely or be pushed back far enough to no longer be a concern.