A grizzled veteran of many 4x's (I liked moo3), I have quite a list of suggestions to keep multiplayer sessions brisk and high tempo.

First, direct control over only 5 systems does not mean that what happens on those other systems is going to stop mattering, you will see players developing elaborate strategies to fine tune those worlds, perhaps going so far as to pairing up with a demand/war buddy to constantly trade off planets as tuning is completed.

You need robust features to dictate to the planet what its intended role will be- a natural chokepoint will need to be a fortress world no matter what its inhabitants inclinations are. When you have enough food, another food world would be a waste. And when you have a hundred colonies, you need to tell them in general terms that they will be a tech colony, and leave the details up to them.

Sometimes you will find natural resource features which you will want to utilize no matter what your empire is currently hungry for- allow the player to set the value thresholds which those preferences will kick in and override the galactic dictates. Or better yet, have it be a point of contention between the planetary governors and their regional underlings.

Suppose there was a planet which was dependent on food being imported, they may be generally unhappy about being in a state of dependency, and extremely unhappy if a massive foodshortage hit the galaxy and they drew the short straw. Or a planet which runs a slight abundance of food may notice the spike in prices, get greedy and decide to convert some of its other facilities in order to smooth things out for your empire.

Or your people could see it as the natural way of things, they're on the central manufacturing hub of the empire, of course everyone is going to bring them resources to build capital ships with. That manufacturing world is going to have political angles of its own, if there is a shortage, the ai running that world will seek out the regions in your empire with the highest untapped mineral potential and entice them to exploit it, overruling your edicts. Or perhaps they would convert some factories into farmland, hindering your war efforts. Perhaps they will even meddle in the configurations of neighboring empires through trade to keep their factories running.

A race with an affinity for research may be upset that theres not even one scrap of research happening on their world and overrule you there too. Or a race disinterested in research would be upset about living on a world devoted entirely to it and flee entirely instead of trying to overrule you.

Its all about exploring the points of contention and deliver to the player the choice of fighting it or going with it. This should also boil over into diplomacy, if one race runs their economy on selling resources to another, but that race decides they would rather just conquer the miners, if the miners don't find a new buyer, they could quickly have two problems instead of one. Alternatively, if one race is reliant on another, the other would have leverage over them.

Next, people can spend alot of time designing and obsessing over ships, you need the espionage element tied into the ship design so that you can tell the ship designer to build a ship optimized for a race you are at war with, or are planning to be at war with, or just want to intimidate. Through their espionage, they should be able to discover your schemey schemes and design a ship that counters your new design. This can get messy, with various flavors of older model ships flying around, sporting all sorts of weapon/armor configurations, but that is an inherent problem for the player in the first place, you need to provide the tools for them to untangle that mess briskly.

Suppose you are playing against a player, they have gone whole hog missiles, you build an anti missile fleet and things are quiet, but then one day they make a 100% beam fleet and then make an alliance with a race whose fleet is 100% kinetic. You see that the kinetic boys are still building kinetic fleets and the beam fleet is still building beams, so you try to get your anti missile fleet to engage their missile fleet, while boldly running away from the other fleets every time you meet them. What should you build? You could build an anti beam fleet, or an anti kinetic fleet, or a hybrid fleet balanced between the two of them. The ui needs to let the player make this choice without them ever having a clue what the configurations actually are. When those fleets finally meet an enemy, they need to know what they are and to act accordingly.

If you demonstrate to the ally who only knows how to make kinetic that you are able to counter them entirely, you may be able to bully them out of the war entirely.

Finally, I love the tech selection system, but is tech trading in? That can really add alot of time to turns, and result in some undesirable power spikes; when someone gets a new tech and they try to trade it off to as many people as they can, before the people that they trade it to have traded it to those others. I mean, yeah, it makes perfect sense, and its loads of fun when its me that turns a breakthrough into 8 more breakthroughs all in one turn, but we are talking about having loads of ai's, dozens of species you are in contact with. If it is going to be a feature, you need to be able to delegate this process to your diplomatic corps and let the ai do the optimization dance-

Only these 2 people have this tech, are either of them willing to part with it? Ok now each of those 6 races over there all have these 13 techs we want, what can we get out of it there? I like these guys, but they trade tech to those jerks, so I'm not going to trade it to them afterall etc

Do this through policies- These are the techs we trade. These are the people we trade tech with, or rather, these are the people whom we want to keep tech out of the hands of. Set the threshold for how much you are willing to give it up for when its a tech that no one else has and for when at least one other person has it. No point giving away your fancy lasers to the galaxy when you can only make one deal off of it. But, if someone else makes that breakthrough, maybe you better cash in while you still can. Those policies should also be a point of contention, both for diplomacy, and for internal politics. (You sold the schematics for my space laser for magic beans! I can't believe you gave the bulrathi infantry-teleporters!)

Edit, tech trading is not in.

http://forum.paradoxplaza.com/forum/index.php?threads/stellaris-dev-diary-19-diplomacy-trade.905407/page-4#post-20586598