They are going to sell a republic DLC at some point, that I can almost guarantee. It will be different (and hopefully better) than how republics worked in CK2, as the reason they are not bringing that content into CK3 is that they are not happy with how it worked. Same for nomads and imperial governments. They would be re-working these mechanics regardless, but they have said that it is easier if they don't build a broken solution and then tear it out when the improved solution is done.
For systems that they liked in CK2 (like pretty much all of Holy Fury), those systems were nearly all brought over into CK3.