1. You can have cooperation agreement cause additional info about the parties in this agreement. FIDS ranking, technology ranking (in diplomacy you can trade only chained techs, if somebody is deeper in one tree than another, you cannot see this), military power ranking... You can limit this only to position in ranking or more detailed info. You can make it a part of cooperation agrrement, but i suggest a new agreemnt for this (something like embassy) or i dont know if this is a part of an alliance (i never was in one :P)

2. Embargo option - stop trading with an empire at peace (free trade slots for an empire you actually like)

3. Make a demanding option supported with threat (declaring war, canceling an agreement...)

1. People have been asking for similar... councils for example....

2. So choose where you want to trade to? And It wouldn't matter too much, you can trade with a HUGE amount of planets

3. There have been variants of this suggested