In terms of making the refleeting period fatal, that could be done, and I like the idea on the surface, but how do we determine what the criteria are for the AI to pounce and kill you, and how is that communicated to the player?
I imagined something like nearby shipyards would be allowed to slowly bleed into threat while the players' "total required metal for full refleet" is below it's current metal storage. Also, the threat would be more cocky, not only doing backstab when there is nobody home, but more willing to engage player's incomplete fleet. Within minutes, all frontiers would fall, the fleet can't rebuild fast enough to contain the lurking/probing threat, and the player(s) loose by invasion.
That wouldn't be the massive AIP death where all your complete fleet on your best defended world cannot stop bajilions of carriers, but a smaller force taking advantage of even smaller defending fleet. The players must to be really stressed and scared of loosing when at 0 metal and half-fleet.
Also, if the fleetwipe was brutal enough, the big reprisal must have already wreaked at least one frontier world in which the lurking, vulture-like threat would join in.
