Interesting! I do find the idea of actual real trade routes tantalizing. Trade routes which can be broken, and damage a relationship between factions indirectly.
Each Alliance type should have its own unique benefit.
- Trade Alliance: Gives access to each others technology in at most, a minor rating. Requires: Embassy, and a Trade Route development thread for each Hex it will travel through. (Dominions, and Skirmishes can interrupt a Trade Route, but can be repaired with a thread re-establishing it either by going around the affected area, bringing back order in the area post-skirmish, or working out a deal with the offending faction in the case of a dominion.)
- Defensive Pact: Factions are allowed four more Ally spots from Pact Member Factions in defensive invasions. Requires: Embassy, and joint development of a(n) outpost(s) in each others regions for each Hex the larger faction takes up.
- Military Alliance: Factions are allowed two more Ally spots from Alliance Member Factions in offensive invasions. Requires: Embassy, and a joint military excersize (skirmishes included).
Obviously in my suggestion a Trade Alliance with a faction three Hexes away with some of the route going through neutral territory would be cumbersome, but it would really stir up some unconventional activity between factions which would be really interesting to see, and would encourage them to expand their borders closer together. Perhaps an established route could even reduce the number of posts need to complete dominions on that path to make it more worth their while. Also, perhaps a major faction can contract a minor faction to protect parts of their trade routes in neutral space, by assisting in the Skirmish, and repairing damages afterwards, and as a result gain limites access to that factions technology.
Additionally, if a very large faction makes a defensive pact, I'm not sure if four more Ally spots is going to cut it, but I'm fairly certain unlimited ally spots is too much. I don't know what else could be provided. Not sure how to involve minor factions either.
Lastly, a military alliance doesn't have as much requirements as I didn't see it needing a significant buff. I see no troubles with minor factions making an Alliance of his sort with larger factions.
Hope that provides some good food for throught!