Right, so if you have build every culture in your world out to that point, where does the Canteena argument come from? You have at least 25 fully formed cultures in your world already, interacting with each other. That is the basic level of DnD, and that seems to be fairly diverse already.
And sure, after getting all of that to work together, being forced to change something could be aggravating, but you might also just find easy places to slide things in. You've already got Stone Giants, Cloud Giants, Fire Giants, Frost Giants, Hill Giants, Storm Giants, Ettins, Ogres, Cyclops and Formarians as your giant races and how they all interact. Slotting in Firbolgs and Goliaths might be a lot easier, since you've built so much existing structure.