I'm going to cheat a little and give 3 x 3 things. 3 big things, 3 small things/tweaks, and 3 things to avoid. Just, I don't have all three yet. I'll update with further reflections.
3 big things:
1. I'd like the transition between the strategic layer and the tactical layer to be more dynamic. Sounds vague, I know. The prime example here was Humankind where any army in the wider vicinity could participate in the battle and you could even move armies into the combat zone to participate, if they were in range. That's probably not going to be feasible with E:R, given how the two game play layers are more separated. But a lesser version that draws in anything within 2-3 strategic map tiles would be useful. You know, so that you don't get caught offguard just because you were busy reshuffling armies or similar cheapo tactics.
And connected to that, a sensible way to retreat before and during battle. From a "real-life" perspective it makes no sense to fight to the last man when ordinarily you'd retreat and save at least some of your forces. Ofc from a game perspective, it's frustrating for the dominant side to always have the weaker side slip away. So, retreat but with a cost.
2. "Endless"-type faction quests that lead into victory types. Honestly, I think that worked so well to imbue lore into the game while at the same time giving your games focus and offering a path to victory.
I would probably also say victory conditions in general and ways to avoid the end-game slog. Or at least make the power levels increase so exponentially that a strong faction can wrap up quickly.
3. Not sure about a big third one and I may change my mind. Maybe a simple one for now: less is more. Fewer research items and buildings but making those that are available more impactful.
The core world-feeder world / cities-settlements dynamic for GalCiv IV and Civ VII sounds like useful concepts to avoid late-game slog. FE had upgradeable outposts, so maybe that can be expanded further or given a bigger role in the expansion aspect of the game?
"3" / a few small things;
1. An AoW 4 and AoW:Planetfall type combat retry. You can let the AI auto-resolve but still take the manual battle afterwards if you don't like the result. There are a LOT of battles in these games. And in particular towards the end, you (or at least I) just don't want to fight every last single one. But I equally don't want to lose useful units just because the auto-AI is too dumb to get it right.
2. At least in FE, please require me to actively select another building I want to construct after I placed one. It happened to many times that I placed a whole bunch stuff just because I was randomly clicking around the city.
"3" / a few things to avoid, for me at least:
1. Don't get too bogged down on unit customisation. It sounds nice and maybe it's a feature many players enjoy. But for me it's often a lot of fairly boring activity for fairly limited gain and even some active downsides. If also all enemies are customised, then that implies I need to study every unit of every army I want to fight in detail to determine how risky that fight is. That is part of a strategy game, but I don't find it much fun.
2. Overly long victory-condition count-down timers. It's frustrating if you (think you) have already won and are just sitting around and waiting for a turn timer to go down, potentially while withstanding a final onslought of enemies. It would be much better to communicate to players clearly and earlier if someone has taken an important milestone and trigger any AI behaviour then. That way, the player homing in on that victory still has one or more objectives of that victory to pursue and ergo something to do other than waiting.
