Ammo room- can store any ammo type , has many decorative details , you decide how big and how much ammo it can store depending on size
Garbage room- purely decorative, just filled with garbage, goes well with the waste compaction room idea by Xavius
pump- a 1x1 or 2x2 or 1x2 room, just has some animated pumps
Data room- a room filled with terminals and computers 2x2 ,slowly uses energy
space bridge a walkway with no roof so you can see the tiny crew walking but it doesnt slow or speed them up
Bathroom- a 1x1 room only with a toilet and tissue dispenser, a varient with a 2x2 room that has a sink, 2 toilets, and a trash bin,a varient 2x2 room has 1 toilet, one sink, and a shower
Prison cell a room that has your enemies captured in it, 1x1 but you can make them adjacent attaching to each other and each block you place will add one person capacity, a prisoner will roam around the cell just like bunkers and you can have a massive cell by connecting them and they will roam around it, you can get prisoners by destroying ships, i dot know how you can implement that but if you do it will be a cool decoration