economic/diplomatc win (aka win without fight)
It would be really graeat to have another way of winning than "get tons of cash, get tons of ships, go for enemy" (singleplayer) or "have someone get tons of cash, someone else tons of ships and go for enemy" (multiplayer). The game would benefit a lot from it, both online and offline.
Diplomacy was supposed to bring new ways of winning the game - there are no signs of something like this coming. You still can only win by building an army larger than yor enemies and crush them. Maybe there are some more ways of doing this with current diplomacy features, but the main concept is still the same. I think it would be nice to have a way to actually win the game without having to build a large fleet - ideally winning "without firing a single shot" (well except for killing defenders on neutral worlds maybe). Some possibilities could be
* a game ending building. it would cost a huge pile of ressources and/or time, also if you start building it all other empires get a message that you build it so they will probably attack you. If built, it would either give you and maybe your allies a huuuge advntage (like +25% damage/hp on all ships, all planets immune to bombing, complete enemy ship and planet vision or insanely accelerated ship manufacturing) or simply give you instant win. this would allow for new strategies in single player as well as in multiplayer where you could have the feeder build the structure while the others have to do everything to defend it. It would be something different from super weapons as it would require civil research only and would give you a global benefit and also make you a primary target - at the moment super guns are usually ignored as attack targets by AI and humn players as well.
* a diplomatic win. This would start with your embassy/diplomatic outpost being able to enforce cease fires in certain areas/for certain times, and could finish in being able to make enemy ships refuse to follow orders or even rebel and turn against their owner. This would also require no attacks from your side. It would also be possible to add a way to negotiate with neutral planet governments to join your empire, making it unnecessary to fight the rebels in orbit. So you would virtually have no reason to get fleet as economist/diplomat. (i think this has already been suggested somewhere).
*economic winning. Besides of the game-ending structure aove there could be things like hiring pirates to help you fighting (see thread on this) or ways to bribe enemies with cash. This way you could focus on your economy, using pure income to defend yourself.
This are just first ideas of what could be done in order to add more possibilities to the players. I think at least some of them they are not that difficult to implement and would greatly increase game experience.