what to do with "bonus room" in rental

[Keith A.]

Input please! I have a 2500 sq ft 3 bed 2 bath 2 living 1 dining and one bonus room rental. I'm having the slab in the bonus room demolished and re-poured in a few weeks cuz it was botched job back in the 1960's. This room was once a 2 car garage but was completely enclosed at some point, windows were added, closets, and doors to the outside and central air piped in. It's always advertised as a bonus shop room/mancave or craft room whenever I've rented it out. My mind has been spinning with ideas lately since I'm having the foundation re-poured, what else can i do to this room to maximize it's income potential. It's the kind of room you would expect to see a pool table in. I've thought about beautifying it into a 4th bedroom. Thought about converting it into a small efficiency, but there is no plumbing in that room. Anyone have any experience with an odd, extra room in a rental? Past tenants have used it for storage of art supplies, or to keep reptiles and whiffle board table. 
Thoughts on maximizing its potential?