Back in GC2 - As soon as you would conquer 1-2 players, there were events such as:-

1:- Terran Alliance would declare war - "On Earth a country named Germany conquered while they just stood by, we wont let that happen"

2:- All Good Alignment CIVs would declare war together - "Your Civ's menace has grown too much"

3:- Evil Civs - They also declare war saying that you are capturing the civs ripe for destruction and hence cant let that happen.

I mean something instant to prevent consolidation of power. The current diplomatic penalty takes too long to kick in. As of now in the first 100 turns I can conquer 10-15 planets without trouble even on the highest difficulties. I play medium map with 10-15 Civs and all abundant stars/habitable planets. 

An alternative solution would be that each civ starts with 15 Legions and assigning legions takes no time/production cost. So the most vulnerable planets can be defended quickly.