I mean there is only a handful of games, that partially try to mimic what Egosoft has done in their X-series - to go basically from zero to Emperor of universe and do and own pretty much almost everything you want.
I'd absolutely love to see games like this in different settings.
===== Underwater setting (think SeaQuest) =====
Submarine Titans (underwater RTS):
https://store.steampowered.com/app/1259 ... ne_Titans/
Aquanox (underwater arcade shooter/sim):
https://store.steampowered.com/app/39630/AquaNox/
X4, but underwater could be basically Submarine Titans and Aquanox combined.
Imagine SeaQuest style world where you can trade, build fleets and bases.
===== Sea setting =====
There could be a lot of potenial here, from historic, fantasy (you don't have to be limited to Earth tech/limitations), to Sci-Fi
I think before X-BTF there were only Pirates theme games, that allowed you freedom of action and owning ships/bases:
https://store.steampowered.com/app/3920 ... s_Pirates/
Sci-Fi setting:
Carrier command series - limited X4 like fleet command:
https://store.steampowered.com/app/6574 ... a_Mission/
https://store.steampowered.com/app/1489 ... Command_2/
https://store.steampowered.com/app/2679 ... us_Rising/
From Depth - open world akin to X4 or Mount & Blade:
https://store.steampowered.com/app/2686 ... he_Depths/
===== Air Steampunk setting (think Last Exile) =====
Steampunk setting could allow very broad range of vechicles (from flying WW1 steam ships, to planes as strike crafts) - if we go with generic steampunk where the world is just empty air with islands flying in the sky, then this is basically X4 already where Sky Islands are just bigger asteroids

Last Exile
https://www.youtube.com/watch?v=m_Mjh0XlXhY
Highfleet
https://store.steampowered.com/app/1434950/HighFleet/
There is very little or no Steampunk X4 games

===== Planetary Setting =====
Here we also have very wide range of possible setting and examples.
Medieval historic/fantasy (from zero to emperor, you can capture own and build castles, cities, manage entire armies)
https://store.steampowered.com/app/2615 ... annerlord/
More leaning to RPG, but you can as well build your own faction with cities, bases and fortresses:
https://store.steampowered.com/app/233860/Kenshi/
This one is an oldie, but it's basically X-BTF, but on Saturn moon Titan (it even has same landing/airlock animation like X-BTF):
https://store.steampowered.com/app/1500540/Hardwar/
More limited in scope, but you can do trade flights between colonial bases:
https://store.steampowered.com/app/6732 ... Simulator/
Frontire Pilot simulator reminds me another oldie - Sim Copter - imagine X4 game, but narrowed to a city/urban area - think X4 with GTA elements:
https://www.youtube.com/watch?v=UIC5pwv86X0
You could even combine Hardwar with Simcopter to a setting similar to G-Police (BTW every time I see X4 station small traffic, I get G-Police nostalgia)
e.g. X4 in domed city on one of moons:
https://www.youtube.com/watch?v=8oYz0zoANjc
Imagine X4/GTA style game where you live in one of those domed cities and build your own faction, own/build building and units, do the missions and trading etc.
Ground based RTS were you can directly control almost all units:
https://store.steampowered.com/app/3016 ... _98_Redux/
https://store.steampowered.com/app/6249 ... Commander/
Another Ground base RTS, but you always still in one tank, still you command other units and build bases:
https://store.steampowered.com/app/4653 ... in_or_Die/
https://store.steampowered.com/app/5140 ... d_Destroy/
The only problem I see with above, would be pathfinding - given that autopilot is ethernal problem in X-series, all those non-space settings would require serious AI pathfinding expertise.