1) Economic Treaties- should provide a 10% economic aid to another power. They should be revocable- but they should take 10% of your economic power. You are giving money. However, having an economic treaty with another race should give each race a moderate bonus to trade/tourism income with each other. Fairly basic economics here.

2) Research Treaties- should allow races to research techs developed by the other race at a discount, and techs one race knows should have a change based on how long the tech has been known the other race should have a chance of gaining for free- based on espionage, at least up the odds of that if a research treaty is in place. If both races are researching the same technology- it should be pooled.

3) Spread of discoveries. The more a race trades, the more they should be able to get free discoveries.

4) Mega event added- one race learns how to pirate a trade good- giving them the good.
other race and original race suffer a massive relations hit.

5) Unhappiness from Trade- the more a race trades- the more it should have chances of rebellions/losing pop- as lesser-skilled workers get priced out of the market. This should happen more to economic powerhouses.

6) Spies on the Capital- gather extra espionage data, don't hurt any tiles, but have chance of triggering event that lowers relations.

7) AIs should try to get allies when they declare war on someone to attack simultaneously. Humans do it when it's difficult.

8) Planetary Defense should have a chance of nullifying/stopping invasion tactics from being used. One way to balance out the ability.

9) Invasion tactics should damage a planet, if 1000 or more troops are used in the invasion. This would stop invasion cheese in either direction.

10) AIs with poor or neutral relations with a dominant civ should team up spies on that civ to weaken the stronger civ's economy. Makes perfect sense.