A very simple way to improve culture a bit
Culture still is a bit underwhelming. A very simple change, which really "fits" what culture already does would be to make hostile culture prevent colony frigates from capturing neutral extractors in asteroid belts and whatnot in the same fashion it prevents colonization.
It would be a method short of permanently committing forces or building a starbase to reliably hold neutral extractors(at the very least preventing vasari scouts from grabbing ones behind your front lines).