Overhaul Warfare, then overhaul trade, then overhaul diplo. Though i feel like diplo should be more integral to the overhauls of the two former.
Right now, there is no way to play as a "Space Venice", where you can become wealthy off the back of trade rather than your own territory. Nor is there any way to really use your wealth as leverage other than by being able to spend it on fleets to win wars.
It'd be nice to see some sort of dynamic system akin to trade flow from EUIV. I'd like to see a system in which some planets or regions become more or less important for trade flow, so owning that area would be a big economic boon, but even empires who don't own that space would have interests in what the trade policies are for the empire that does own that space.
One idea that seems like it might be cool is to have common markets that empires can sign up to. Imagine if empires could organize for food to be an actual galactic resource, shared between all participating empires. You could have empires which specialize in becoming a bread basket, who can trade off their food to other empires, who in turn can then afford to go all in with energy or minerals etc.
The possibility of interdependent empire builds not only gives diplomacy some interesting hooks, but also allows for more dynamic reasons to form defensive pacts etc. "these purifiers aren't a threat to me, but what if they attack my bread basket ally?", as well as setting up a system where trade sanctions could actually have some weighty threat to them.