The kind of game I'd love to play or develop would be a realistic space tactical game.
Read the Honoverse series from David Weber for a "navalized" version, or if you're into just a "bit" harder sci-fi (like me) read the Lost Fleet series from Jack Campbell.
What I'm looking at is a game that focuses on fleets instead individual ships. Instead "fixed points" you'd set velocity and heading for fleets, naturally the game would be played on a light-minute scale, and "battle (relative) velocities" around 0.1 c to 0.2 c. You should really read a Lost Fleet novel to get what I mean.
It would be all about timing and getting the right angle on your enemy. If you can brush against the edge of his formation with your own (like a file rasping on the thin edge of knife) you could do enormous damage through your local superiority.
I forgot to mention: no in-battle manufacturing, no "purchasable" reinforcements. Only objectives and a limited set of ships (or fleets) to achieve them.
The whole solar system would be a battlefield. By the nature of things one party will be in the system before the other. We shall call it the "defender" (even if there is no objective to defend in system, for the sake identification we will do so). The one to arrive later will be the "attacker".
If FTL is used to arrive in the system (and this is a given if we speak about interstellar warfare)the attacker will always have the advantage thanks to the light-speed lag. When he arrives, his "sight" will begin to travel in spherical pattern to all in the system. The defender who has already been in the system for a while on the other hand already has a lot bigger "sight print" since it had more time to expand. Therefore the attacker will detect the defender and act as he chooses , while the defender will have to wait for the attacker's "sight" to reach them and can react only afterwards.
The game could get really hairy if we incorporate this lag into commands too. This would mean that the further your fleet is from your command ship(s), the more time it will take to react to a command. This would mean that you would have to give commands in advance and hope the situation that develops doesn't make them downright stupid when the time comes.
This makes the "approach" of the enemy fleets a wild guessing game. If I change course, it will take time for transmission to reach all ships/fleets. When the fleets (probably at preset time) execute the maneuver, the enemy will once again have to wait the light speed lag (which is more for him, since he's farther away) and can react only afterwards.
Let's give a simple example:
I'm the attacker, and I approach in a simple flat box formation with the smallest face of my box facing the defender. Our objective is to eliminate the other fleet. The defender also uses the same formation. As we're only 2 light-minutes away, I give the order for the whole formation to pivot upwards. After a while the defender finally sees this and pivots his formation even further up, so when we "collide" his steeper angled formation will file at the top of my own....but he was suckered. 0.8 light-minutes away I already gave another order to pivot the formation back down.... and now I will file at the bottom edge of his formation, and he sees this too late to reposition his formation.