I'd say play a map without preset starts, especially if we're doing something like the above-proposed VCs, which I'll confess I'm not overkeen on. If you're really set on capital-induced death, have you considered cumulative VPs? Set it to, say, 12? For 6 players, that'd give you a game between 24 and 144 turns (though it'd be quite unlikely to clear 72). Losing your capital is a blow, but not fatal by any means, and it gives clear warning for everyone who's at risk of losing to gang up on the leader if they're really dominating. Or go with 18 VP if 12 seems too low - that's 36 to 216 turns, and would still likely not clear half the max.
Still, even with this, it puts a massive emphasis on rushing (and commando raids in early summer, natch). I cannot put into words the extent of my loathing of blitz games. Honestly, I'd be happiest with no victory conditions. If it's plainly game over, those who are on the receiving end of a steamrolling can concede. Creating very structured, artificial victory conditions is just begging someone to game them to "win".
Hmm... looking over the maps, I'm seeing Hyborian, and (the exceedingly ugly) Lonely Mermaid ("less seas" version) as being Bering-sized without fixed starts. Pangaea, Random: Hellscape, Sha Bay, and 3Isles (Good or Lots of Water) are also close-ish in size. Yalun-To would do for a slightly smaller game if we had 3 people wanting water nations, or a closer-to-even sea-to-land balance between 4 terrestrials and 2 aquatics.
[Edit: I'm willing to switch to a water nation if we need another for balance. This presumes MA wins the vote, as it's on track to, obviously.]