i started a 10 player realm with friends and we each a doing a a large scale builds and im having trouble of thinking of theme i can go with on a large scale i have a lot of space to build but i cant think of anything
Do an inside-out building theme. Anywhere that you would have a building, you can put up the walls but you cannot put a roof on it. Anywhere that you would have outside-stuff (greenspace, pasture, crop farms, etc) you have to put some sort of covering on. I found this one tricky, I was constantly underestimating how much space I would need for any particular area and ended up with really weird shapes.
Or you could do a platformer concept. Each function you want to install into your base has to be raised up on some sort of platform.
Like parkour and wish more people would get into it? Build a vertical base where you have to parkour your way to everywhere.
If you're into the completely silly stuff, build all your stuff sideways or even upside-down to the fullest extent possible. One nice concept I tried was based on "the floor is lava!", everything was hung from the ceiling and the only way in was porkour (this is not a typo, by the way).
Total war, XX-XXI century.
Housing in tanks, in trenches, in green-woolen tents, in bunkers, furnaces built in non-functional (that's optional actually) guns, ranged weapon abuse, craters and vehicle wrecks here and there, pillboxes with shooting slits, "enemy trenches" left darkened and with subterranean water-soulsand-elevators bringing additional mobs into them from underground mob spawners.
Need a bunch of designs? Here's appetiser: