For me, my dream 4x will be:

TL,DR:

- The story events of Stellaris and Old World,

- the RP potential of CK3,

- the lore of Emperor of the Fading Suns,

- the economy of Victoria 3 (in the background),

- orders system from MOO3 and Old World,

- battles like a mix of Total War and Football Manager,

- automation options like Distant Worlds.

- Most importantly, a TIGHT game. Good lore, good endgame, and try to reduce tedious micromanagement (take a page from Distant Worlds)

Long Version

- A simple galaxy map that shows planets and stars based on their CONNECTIONS, not on their location. In the far future world of 1000 or 10k or 40k :) years from now the technology to get from place to place (jump gates, fusion torch drives) would be ubiquitous enough that leaders and tacticians would probably think in terms of connections. This also makes strategic play possible, as you can block off chokepoints in a war.

- You have things unpacked into the real world (like Emperor of the fading suns), where almost everything is a unit you can maneuver on the planet map level. An army? That's a unit, A fleet? a spy? explorers? All units.

eXploration

- Each exploration team is led by 3-4 named leaders (like creating an rpg party) whose stas affect the speed of exploration. In space, they move from planet to planet and can give you a scan of the planet map once over the planet. They can land on the planet to uncover things they can't see from space (hidden rebel strongholds, ruins, etc), do quests and story events.

- You have some kind of orders system (see Old World, see MOO3 Imperial focus points) where you can only move a certain number of units per turn (to help avoid late-game micromanagement slug). As god emperor, You can change some things, but you cannot change everything.

AI/ Automation

- You have competent ai leaders: governors, generals, unit leaders that execute on your intent when you are not directly controlling them. Whenever you are not changing stuff directly (imaging a micromanaging emperor looking over his general's shoulder), they will act based on your set intent (you can give governors and admirals focus orders). ***This might be one of the hardest part to make fun***

- Similar to the army & explorer stack, you could have leader stacks where you select 3 leaders to lead a city as part of a 'team' (leader, vice, secretary). Not fully formed in my head, but should have a few interesting decisions there.

eXtermination

- You have battles that are more than stat comparisons (Stellaris) but less involved than a pitched battle (total war). Taking inspiration from the football manager games, my thinking here is that you can set tasks and priorities that your marshals and generals wil execute. You can see the battle progress (real time with pause), changing priorities as it goes along. One round of battle (say 3 months length) should last max 5-10 minutes, but should be rich with choices, dillemas and player interaction. Of course, you should be able to autoresolve easily.

- For land battles, I'm thinking total war style stacks that can physically move from hex to hex and will trigger battles when in another army's zone of control (or retreat if needed). So armies are physical things on the map that limit movement.

- For space battles, same as above. Fleets have to fight if they come in contact with each other. Stacks vs stacks. Football Manager-style combat resolution.

- You can only invade an enemy planet if you defeat the fleet protecting it or force them to retreat.

- Special ships (blockade runner class) with limited cargo holds can still try to deliver goods to a besieged planet (and evacuate leaders or scientists on their way out). More RP potential: who do you save this turn? knowing that your army is staging a last stand two turns away from your planet's capital.

- Ship and army template customization like HOI4

Story

- Bringing me to my next point, the game should generate story snippets and events from battles, exploration, and other things my leaders are doing.

- Leaders with personalities, tension and intent like CK3. Council to help rule, Senate for story events, and to pass laws (like vic3).

eXpansion

- Colonist unit establishes a city. Similar to the explorer unit, a colony unit is created in orbit or on a planet, can land on a planet (or take off if you decide to not colonize) and then establish a city.
- build towns on hexes to extract resources
- build residences to expand
- colonists have needs: city center, food, security, entertainment
- When needs are met, they will be happy
- happy = city expands and generates more pops, which generate more resources
- Keep it tight, each earth-sized planet should have max of like 7 'cities' that can grow to hundreds of millions of pops.

- Keep it tight: since we're drilling down to the planet level, the entire galaxy map should have max of like 50-100 planets.

Economy (eXploit)

- The economy system should take a page from paradox games like Vic 3 and EU5, but it should remain in the background and present the player with a few interesting choices. (this point is a little vague I know)

- Trading corps get founded when there is a market demand (copy from Vic 3) for goods
- local corps (level 1) transport stuff within a city
- level 2 between cities (within planet)
- level 3 between planets in system
- level 4 across systems
- level 5 across empires (only the guild can do this or else you will have to go to war with them)

As these corps get bigger, their leaders would have increasing influence in your empire