I use the option to speak on behalf of someone I have a free trade agreement with all the time, really helps keep things calm when aggression is set to chaos.
Regarding the OP explicitly multi-lateral diplomacy is an interesting and ambitious idea. But in my experience the game already has some pretty interesting power bloc dynamics. Scrolling down the list of empires usually reveals a pretty good webs of wars and free trade agreements. But is it kind of messy like system borders. Personally, I like being able to be enemies with the enemy of my enemy, enemies with a friend of a friend, friends of my friends enemy etc. That ability is sorely missing in lots of diplomacy systems. If there was some kind of graphic where all the connections could be seen at once it would make it much easier to see all the different power blocs. I have seen that in pvp diplomacy systems that otherwise aren't more sophisticated than SW and it usually works quite well, especially if the nodes rearrange themselves based on the relationships they have. I think I have even seen it in a 4x but I forget which one.
There is of course a lot of room to extend the diplomacy system. Top on my list would be adding 100,000 and 1,000,000 options to the cash part (or better yet allowing us to just type in whatever we want). The ability to only offer disputed bases and colonies that are in what is regarded as the other player's territory is limiting. So is only being able to demand ones that are in space regarded by the game as yours. I'm not sure if the ability to buy or sell any tech level rather than just the most current would mess up how races with special techs currently works (though there seems to be special mechanics dealing with trading those anyway) but it would certainly add a lot more flexibility. Being able to trade ships would be somewhere on that same list of suggestions.
I am starting to learn how to really bend the diplomacy system to my will. Going to war for just long enough to grab a planet then bribing for peace and swapping the planet back for a tech is a great way to get to the cutting edge of technology at the start of a slow research game. It is also much cheaper than just buying the tech off them. I never even give away my territory map now, I just pay the AI for theirs. If they offer to swap territory maps with me I can usually just open the diplomacy screen and get them to give it to me for nothing. Using the suggest setting on diplomacy (I play with suggest gifts, treaties, war and trade sanctions) to allow me to speak on a friends behalf is part of the mix as well. I'm not 100% sure what the relevant automation setting is, but it would be a simple process of elimination to work it out.
Mothballing ships is another interesting idea but it would have to be worked out properly. You don't want mothballing and unmothballing to be too quick and easy or there is a temptation to always mothball something if you will not be using it for even just a brief period. If the ship can't move until a construction ship comes to unmothball it and it requires a visit from a construction ship to mothball it in the first place that would go a long way towards balancing it.
Well, I'm thinking that to bring a ship or ships out of mothball would cost some resources and/or some cash. Also, it would take a turn or so to bring the ship(s) out of mothball.
This idea came from Space Empires V....anyone remember this?
"Tho' much is taken, much abides; and though we are not now that strength which in old days moved Earth and Heaven; that which we are, we are; One equal temper of heroic hearts, made weak by time and fate, but strong in will." -Tennyson