This is going to be just a load of random ideas for the game that I think will probably be coming after humankind since endless legend has endless space 

with paradox games you can play as rome and lead your empire to conquer the galaxy in Stellaris. Yes this also happened with endless legend endless space but I feel like with humankind, once its finished there will be the opportunity to make a better Stellaris competitor. Stellaris is the game I have the most hours in total of any of my games and I feel some of the mechanics of humankind such as territories and ethics could make a better galaxy spanning game than stellaris or endless space. Territories could apply to planets making planetary management more interesting as each planet could be managed like a small humankind map could be played. It could also apply to star clusters, there are many ways that territories could be applied to a map of a galaxy such as the galaxy map being nodes/territories of star clusters that would be played in like endless space. Or you could go all out and make the galaxy map like no mans sky with a section of space being classed as a cluster that would be fought for and then from the capital of the cluster you would manage the systems in the cluster as a list or it unwraps the 3d cluster to a 2d layout where it could be managed like endless space, another game you could take inspiration for this from is slipways where instead of planets you would be linking systems which could make for an interesting cluster management system.

A big mechanic of Stellaris for me is megastructures and since dyson sphere program released I've thought about how the construction of the dyson sphere in dsp could be applied to other structures like ring worlds, star lifters or even as far as matrioshka brains in a game like stellaris, the slipways system of cluster management could apply to this to allocate resources to projects efficiently and to make megastructures commonplace in the galaxy. a popluar mod for stellaris is gigastructural engineering which could provide some inspiration for the types of things that could be built within a system. yes these structures will remove some of the challenges of economy management as you will probably have more than enough of everything but stars dont burn forever especially when you're robbing them of their fuel and structures dont stand forever especially when built around a burning ball of plasma that could shoot a flare at any moment. if your economy is built on one dyson sphere and a star lifter and they get damaged or destroyed then your not gonna have a lot of anything. megastructures dont always have to be massive star spanning structures that create the tower on which your entire economy balances they can also be smaller structures around a planet or orbiting a planet such as a planetary strip mine built around a planet or an engineering testing station built in orbit 

warfare in stellaris and endless space is fairly basic with fleet vs fleet combat but a planet could be rendered in hospitable by a relatively small rock and gravity is a useful force to use against your enemies. a sufficiently advanced civilisation that could build a dyson sphere could probably also move planets around maybe even attach thrusters and warp drives to them and launch them at lightspeed to collide with an enemies planet or even to cause a big enough gravitational disturbance that could throw a planet or unsuspecting fleet out of orbit. yes you can have your fleet combat and invading armies if you want to keep your enemies infrastructure and population but if you can launch a planet at your enemy then you can utilise the rubble of their civilisation to build a ring world for your own people. this would be a useful system against less advanced civilisations but equal or more advanced civilisations could use your own techniques against you and each of you could probably blow up a planet and redirect it if it was coming towards you, this could call for more stealthy techniques like some kind of weapon that could be shot at a star to blow it up without your enemy noticing. you could also introduce concepts like subspace which could allow you to move a ship undetected into enemy territory and surface it into regular space to attack before submersing again. planets could also have their own ground defences such as building a gun into a mountain that could be aimed at an invading ship in orbit to blow it up or mac cannons from halo in orbit for more range. you could even go all out and weaponise megastructures like dyson spheres to launch a beam of energy.

ethics in humankind have made each empire much more interesting and this concept could be taken to a stellaris like game where the ethics screen is available when designing the empire and some of the options are available that would allow you to choose between authoritarian and democratic style governments, your stance on nukes and more dangerous weapons, your religion which if you want one will allow you to design some aspects of that. but as you advance the game there would be choices such as how you treat alien empires and how you treat aliens within  your empire (theres a difference between enslaving your own species and other species), your stance on megastructures as you could be concerned about the environment of the galaxy and feel that megastructures are polluting space and are disturbing the natural order of things

eras are a very fun concept in humankind that wouldnt really fit in a stellaris like game as you dont know the civilisations that will exist in the future but the concept can still exist with empire types available based on ethics, keeping the fame system and era stars based on achievements such as population, megastructures, planetary districts and infrastructure, planets and systems, money, influence, ships destroyed and technologies discovered. you can also take the deeds system for things such as building specific megastructures, discovering specific technologies or doing specific things, gigastructural engineering has an achievement system that could take inspiration from as well. the influence system should stay to slow the players expansion allowing them to claim systems at the beginning and then to claiming clusters. megastructures should not be wonders yes it should give you fame when you build one but building subsequent ones shouldn't give as much until they are just built like normal districts. the administrative cap from stellaris would be useful as well as influence to slow the players expansion as megastructures would require people to manage it and so would take administrative capacity to build and run.

cosmetics are also a pretty important part of stellaris, ship sets, city designs and species are things a lot of people spend a lot of time choosing and so with the customisation available to player character creation in humankind there should be an equal amount for ships, cities and species allowing people to make what ever they want