Sorry for the delay - I meant to answer within a couple days, not more than a week later :/ (had some urgentish family stuff to sort).
I think you both make some good points on real hurdles (for the concrete example of shared Multiplayer servers). I wouldn't lean out there and say they can be overcome, but it was
just a specific suggestion - and while there are other hurdles for other things, I'd still be optimistic that it can work in some areas. Perhaps I need to spell out some more of my (long term) ideas :).
( P.S. I quite like the epic "Great Server Split" ballad :P )
First, happy to hear you're open to the idea, if (understandably) reserved regarding investing time into it. (wouldn't make much sense for me to want to carry things further if there's no interest in the first place).
Totally agree with this sentiment: "I just think that the most polished experience will come from games presenting themselves as standard standalone games, and that large games should strive for this level of polish. "
I'm guessing one of the levels of cooperation I'd be looking at (to spell out one of the long terms), is something akin to eg the Battle-Net client. I've realised over the week that this would have a number of significant issues though (esp: either you'd be losing Traffic / visibility from Steam, or you'd need to deliver on 2 content platforms, which is certainly a horrible solution as well :/ ).
So no, I didn't mean for Zero-K to bundle BAR, or vice versa (but I guess a "Humble Libre Bundle" offering BAR, Zero-K and 0AD might be a nifty marketing gag :P ).
I guess the two main angles I'm thinking from are: Increase visibility (Communities, let alone players, are often not/hardly aware of other great Open-Source RTS titles)
Creating a space for shared discussion (+ potentially joint projects).
I guess more from me when I've gotten more feedback from other communities (I've only asked / got replies here and on 0AD yet..)