- Supply Help : use your stockpile transfer ability to fill someone else's stockpile. Might require supply connection between capitals?
- Research Partnership : halves (or diminish in an other way) thee research cost of tech your partner has. Can see the information about tech researched and being searched (even without debug option, through the pedia page I guess). Possibility to have it one sided?
- Shared boni : Allow empire to share growth bonuses. I guess home-world bonus should be reduced for the receiving empire, and resource bonuses consume one resource per empire. Might or might not be applicable to other empire wide bonuses. There are some thought to give about failure to honor some agreement, I can expand on it if you want.
Research partnership would be a default include for alliances and would weight into the how attractive potential partners are. Being too liberal about research partnership might not be optimal but can't be too risky. It should greatly reduces the tech gap between dominating warmongers and others. The only issue I see is that it tips the balance between science and industry focused empires (even more) in favor of industry (the industry focused empire can have half price researches and rely n other to make the hard work of tech breakthrough), but with complex enough diplo trade science for industry should be doable (at worse, science poor empire could be looked down by others when partnerships are considered). One pitfall to avoid IMO is tech brokering, were one faction buys a tech and immediately sells it to the rest of the world, for both profit and even more tech to broker.
Supply help could be used by AI in any of the following cases : in exchange for some other kind of per turn partnership, toward the weakest empire when fighting a common enemy, from an empire with crippling fleet upkeep toward one with low upkeep again when facing a common threat, and also toward the AI the closest to the front.
- Have unhappiness generated by distance to palace (affected by fastest hull and engine tech, like colonization) and number of colonies.
- Have a way to buy happiness with other resources. Could be through -expensive- buildings like Gaïa transform already does, through techs, through PP/stockpile upkeep, or through influence if it makes it to the game.
- Have happiness affect PP and RP yields. For example, PP totally wasted at 0 happiness, 80 percent wasted at 1... and fully functional at 5, not applying to concentration camp and maybe some other bonuses. RP could be the same but more demanding, with no prod at 5 and below and full prod at 15.
- Have planets with both current and max happiness below 5 (or some other number) prone to rebellion (whatever rebellion is, creating "neutral" or an entirely new faction).