It seems to me like at some point using ships is going to get ineffective. You can put engines on planets, and you can put enormous guns on those planets the size of several ships, and every planet is fully capable of containing its own self-sustaining life support system. Structural integrity is rarely a concern because its own gravity does everything and more that any number of struts or wires could and requires virtually no upkeep in how they have room to develop their own resources (particularly if you dig down below the surface and utilize every inch of volume the body provides). You could even utilize the planet's core and use it as a gigantic generator if you so desired. It doesn't cost any resources cuz the whole thing was right there to start, and anything you need can just put strewn across the surface: fire control, environmental alteration mechanisms, heat dispersal units, resource distribution, elevators to and from the surface, hangers, crew quarters/command...Hell, you could even put a shield on one of them if you wanted to.
It'd take a long time, but there will come a day when ships are just plain not necessary, because everything they require can be put bigger, badder, and more efficiently on a planet. Ships will literally be replaced by planets, and this is something that I want to see emulated in game. Because why waste my time building a fleet when I can put guns on a planet and bombard other planets with it? Or chuck its own moons at other planets to smash them like vases? Or pick up and double gravity slingshot asteroids around itself, then around the planet's sun, with a final violent rendezvous with the enemy homeworld?
And this then begs the question: how big can we go? At what point do we dock planets to other planets, or otherwise link them together with interplanetary space elevators? When do we go from constructing planets to uniting entire solar systems to treat as a single entity, and then weaponize/utilize THAT construction? When do we get to move entire solar systems through the galaxy like one big space ship?
Perhaps it never ends. Maybe we can build up entire galactic networks and create a giant ship out of the entirety of a galaxy. We can already collapse stars and utilize their fusion on smaller scales, why not make more? Much more efficient ones, where mass doesn't hamstring its ability to project the monstrous power from its core, specifically engineered to make use of every last bit of power its capable of putting out. When can we make gigantic fusion generators out of suns?
The sky is the limit here. But at the very least, I would like to see plantary battleships. Why make cruisers with nukes when you can just lob asteroids at enemy planets with the power of entire fleets combined using nothing but gravity and velocity?
That's my suggestion: get bigger. We've got universes to conquer here!