A couple of things I think often are forgotten when discussing whether CK3 automatically would have too little content or whether cutting a certain thing would be sensible and would ensure that it is feasible to add everything else: Different features in CK2 aren't necessarily equal when it comes to the difficulty of implementing them, meaning a certain cut might not be enough to make CK3 feasible, and some things could be cut from CK2 without you really noticing the loss (at least assuming you're playing with all DLCs), meaning a certain cut might not impact how enjoyable CK3 would be on release.
Regarding the first point, Sunset Invasion is often listed as something people want cut or would expect to be cut, but aside from the graphics (which presumably would be overhauled across the board in a hypothetical CK3) and the specific localization (which presumably would be overhauled across the board in a hypothetical CK3) that DLC is tiny (about 50 events in its own event file, and perhaps 50 more elsewhere (I believe that even covers the Bloodthirsty Gods stuff as a whole, which non-Aztecs can benefit from)) and I don't think there's any functionality that Sunset Invasion uses that wasn't in the game before it was added (if it was part of the initial release) or used elsewhere (if it was added later and Sunset Invasion took advantage of it). Assuming the mechanics for the Mongol invasion in CK3 would be reusable for the Aztecs (or another, similar invasion; perhaps the descendants of a lost Roman legion march in from the south or something), redoing the release version of Sunset Invasion wouldn't be particularly difficult, and at least some of the extra bits (e.g. courtiers joining a holy order with certain generic holy order events) would presumably be possible to copy from somewhere else as well.
Regarding the second point, there's a bunch of stuff in CK2 that's not used if you have a certain DLC -- e.g. the non-Conclave education stuff if you've got Conclave active -- or that has been replaced with a new system -- e.g. the personal combat system used before 3.0 -- that you'd not have to create two new versions of. Just how much work that would save is difficult to say (particularly since not everything is equal, since there are files that are partially commented out or partially disabled with a DLC, and since the use of scripted_triggers and scripted_effects means something could be much larger than it appears at first), but just cutting the non-Conclave education events, the lover events that are disabled by WoL, and the non-tOG peasant revolts would reduce the size of the events folder by about 2 %, and there's definitely more stuff than that that could be cut (e.g. the Golden Horde/Ilkhanate invasion events and the aforementioned old personal combat system) without it being noticeable (again assuming you've got all DLCs; you'd obviously notice the loss of non-Conclave education if you don't have Conclave and don't get a replacement).
