So I wanted to put all my thoughts on culture in one spot, as its a hot topic right now...
Let me preface that I understand it may be too late for this to be done, but thats irrelevant on Principle.
I like how culture spreads, i like the advantages it gives you in increased income and even repair rates, I think some great advantages for it should also be an increased resistence to bombardment and surrender.
Instead of flipping planets, Culture perhaps should make it possible for enemy planets to 'surrender' and flip when they have been under bombardment for a significant ammount of time, the chance of surrender could be increased by virtue of the scale of the bombardment as well. Culture would also defend from this tactic, it would push the game along well.
But I would take out the ability of culture to actually 'flip' a planet, instead culture that bypasses enemy planets could decrease production times, tax rates, growth rates, and generally make your planets more and more miserable.
This would make assasinating culture centers right away not a big issue, instead you would just need to build your own which is fairly quick. Losing the culture war would never lose you the game directly like it can now, if your big and strong enough it wont matter. In exchange for this new system culture could spread out farther/faster because it will never actually flip enemy planets, just reduce their income.
I think this is much more realistic, and there is allot of people who are kind of annoyed with culture being such a direct weapon as to convince your Homeworld to flip! Frankly this is more of a 'Containment' strategy as it could seriously dehabilitate an empire by reducing its income and stagnating its population growth as well as making your invasions much quicker and successful.
The difference is you would still Need an invasion!