I'd love it if they opened up the game to modding.

I don't expect them to fill in the end game with too much stuff. Regenerating monster contracts would suffice. As long as there is a modding community, UGC should fill a void in terms of quests for end game content. It's not too important, because the game is narrative focused, and hopefully the community can flourish the end game stuff for those people that really need to keep playing (like me).
What I really want is New Game+, where everything is scaled to be far more difficult, no matter your level or equipment.
Use the difficulty setting to adjust how much stronger enemies are:
1 enemies/quests same level as Geralt.
2 enemies/quests 1-3 levels above Geralt.
3 enemies/quests 4-6 levels above Geralt.
4 enemies/quests 6-9 levels above Geralt.
I can't see them adding too much of your wish list of stuff, outside of the cosmetics. There will be Triss, and Ciri alternates, and Gwent cards from what I've read, leaving 9 DLC. I can't fathom exactly what they could do with that, but it has piqued my interest. If the Mine Quest DLC is an indication, then we can assume there won't be very large storylines coming out of this program. (*spoilers*) Sure, Kaer Morhen feels empty, but that's because it's dying and abandoned. I don't see a need to revisit it, as a lore based thing, it's supposed gone off. It doesn't make sense for them to revisit that location. We explored it enough in the first Witcher game, and it's in ruins now.
I agree, there should be a place call home with your partner,
with a storage case, I just don't think Kaer Morhen is it. There are a several locales that I felt could be interesting to inhabit that were far enough out of the way, but not too far off the beaten path.
As for end game, this is a narrative driven game, so once you've completed the main story line everything else is going to seem/feel far less important unless you left some of the more interesting side quests to the end. There is only so much content CDPR can generate in a timely fashion. They've already promised to release 2 expansion packs, so I'm going to assume that their time will be occupied.
I would rather CDPR focus on their Season Pass stuff and when they're done those
hopefully they can look into more interesting end game DLCs that are bigger than these free DLC they're making (miners quest is short).