Lots of awesome thoughts here, thanks! I should clarify our history with RTS games:
Age of Empires 1-3 (and all expansions)
Empire Earth 1 and 2 (and all expansions)
Civ IV and V (and all expansions for both)
Rise of Nations (and all expansions)
Rise of Mythology (kind of yuck -- and I'm pretty sure that was the title; anyway, it came after RoN)
Supreme Commander 1 and 2 (and if there were expansions, we had them as with all the games)
There was also something that was really good but less known... Empire-something, which is vague. I would have been right after Empire Earth 1 for us. I mainly remember tanks.
We also played with a lot of various mods for a lot of these, and a lot of custom maps. Sorian's AI Mod was actually something modded even further, and then after frustration with that I wound up making AI War, heh.
And stuff they played but I skipped:
Starcraft 2 and I thnk the first expansion
Star Wars: Galactic Battlegrounds 2
We also played some other non-strategy games:
Neverwinter Nights went over pretty well, and we played a goodly bit of the platinum edition as a group, plus some custom stuff.
Diablo 3 I skipped, and my dad and uncle were pretty bored with, but my uncle's colleague was really obsessed with.
I also played dozens of others on my own, but that was our history as a group.
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From what I've read, CK2 doesn't really sound like a good option for co-op because of the way one player can get accidentally subsumed in another.
With EU4, it's exactly the sort of game that would be a no-brainer for our group to jump on if I was there. Possibly my uncle's colleague can pick up my place with that game, but it depends on his interest.
Every one of them is capable of delving into this, but it's a question of time and motivation, really. My dad and uncle basically want to hop on two nights a week, spend 30 minutes chatting and then 3-4 hours gaming, and that's it. No solo play or outside learning or anything of that nature. For me, I could never just do that, I always wanted to explore things solo, so I'd quickly wind up being the expert. Of course, now I just frankly don't have time, and they play on Friday and Saturday nights, which is one of the few relaxation times for my wife and I, so that kind of conflicts.
I looked at SoaSE back in the day (before making AI War), and I passed on that because I felt like the pacing was too slow. Hence why AI War is faster-paced. I know they've added a lot of cool stuff since then, though, and the slower pace was really more a personal issue on my part rather than something that would bother my dad and uncle. They are absolute space nuts (both in terms of sci-fi and actual science), so anything in space is a plus with them. But they're also moderate history buffs, so that also goes over well. They like fantasy insofar as LOTR goes, and not really much beyond that, though they'll tolerate it for a good game.
We had a ton of fun with AOE2 back in the day, so part of me was considering AOE2 HD. It has been more than a decade since we played it, but I still do worry that it will just be too familiar and run out of interest too fast. Man that was a fun game, though.
I remember looking at Kohan back a few years ago and not being impressed for some reason. I forget exactly why, at this point. I would always play a dozen plus demos every time it was time to choose a new strategy game, and spend an hour or two with the demo before deciding if it was right for the group. Usually I'd write up an email of the top two or three options and then see what people thought, but a lot of the time it boiled down to something more unilateral on my part as they were just like "tell us the good one." That's been more and more the case lately.
They absolutely _adore_ games that go on for tens of hours, same as me, so EU4 would be ideal for those purposes. But in reading about things, I think that SoaSE: Rebellion looks really good (actually it interests me now, too).
Thanks for all the thoughts, folks! I'm bummed I missed the EU4 deal on Humble, I had been looking at that, but I'll see what I can find.