I think what we need some shake-ups that would encourage us to revisit some Civs. What they did with Norway or France Catharine's new persona is a good example. I don't know if I can point any particular Civ that badly needs rework. There will be always dull, straightforward, or weaker Civs in a game if you have them over 40.
Very true. Additionally, there are some little things that would ripple throughout the civs, like maybe giving siege units a bit more defense against attacks from walls? That would have large ramifications, maybe making the Ottomans need a slight nerf, but it would incentivize actually using siege units before the industrial age. It would make the AI cata-spam a bit more terrifying. And it could change the way light-cavalry civs spend their time, given that currently their strategy is "Destroy your opponents before they invent walls. Get the stragglers in a few thousand years while you finish your hydroelectric dams." (Exaggeration)
Anyway, I'm not a master of balance changes, but I think the point stands. Another way to look at it is to compare current Canada with 2016 Canada. I don't recall one specific change they made, but they are a lot more viable to play as now, especially if you don't play using the (admittedly most efficient) "kill on sight" principle. Of course the fastest way to win any victory is to destroy your neighbors. Any victory... except culture. And if you're Canada, there are actually really good strategies out there, like rushing the Eiffel tower and national park spamming (for free). But again, a small change, like increasing the tourism provided by national parks, could make Canada an actually really good civ, make national parks more viable for other civs to chase, or could make culture victories as a whole more solid, more worthwhile.