Why not a system who needs some points of research (a few percent), for each technology completed (and owned by trade, invasion, etc.), a sort of cost for knowledge maintain.
Each technology owned, decreases slowly the research capacity.
The player must invest in additional laboratories to maintain its research capacity at the same level.
Example:
Research capacity: 1000 points research.
Research cost of technology: 1000 points research.
Cost of maintaining technology: 10 points research (1% in this case).
Research time is therefore 1 week.
Once the technology is completed, it costs 10 points research to maintain research knowledge.
The research capacity becomes: 1000 - 10 = 990 points research.
A tag is added to the XML file in order to provide values more or less important for modding.
Example:
Culture ID="InterGov"
DisplayName Interstellar Governments /DisplayName
Cost 200 /Cost
KnowledgeCost 2 /KnowledgeCost
Culture ID="InterGov"
DisplayName Interstellar Governments /DisplayName
Cost 200 /Cost
KnowledgeCost 10 /KnowledgeCost
Just an idea with my bad English.