I would reaquascape into a ‘pillars and arches’ style. I think it adds a lot more flow. Basically, you build pillars that are different heights/widths and add arches between them. Watching the fish flash in and out is really fun!
Angelfish will always keep you interested! All of the centropyge (dwarf/Pygmy) angels will be happy in a tank that size. A mandarin would probably be happy in there as well, a tank of that size and maturity could probably sustain one with very little assistance. Blue line pipefish are gorgeous but specialized eaters. Since you have the cow/box fish already I think it’d be fine though.
Start a zoa garden! Always something to do with a nice zoa garden! Add in some rock flowers to break up the zoas here and there, maybe even a pebble of Xenia for movement!
Get more inverts! Fire shrimp are my favorite because I have trained them to be very sociable with me, swimming across the tank to ride on my arm, begging me for food, etc. Plus they’re beautiful and will give your fish an occasional cleaning. Have you checked out reef lobsters? Very cool critters!
If your corals are boring, add movement. Euphyllia and anemones are great choices to create some sway in your tank! A cramped hammer garden is always a knockout when visitors come and look at your tank! A gorgeous RBTA (or whatever color suits you) with a couple clowns is always a winner.
Starting a refugium is always good. It helps your tank and it gives you a nano tank! Like kenny90 already said, you can put some really cool species only in there! A mantis fuge, angel let, frogfish, sexy shrimp squad, pom pom crabs, basically anything too small/specialized for the main tank.
I hope this helps! I get it. Every now and then we need to shake things up and now is the time to do it! Go through and frag/sell outright any coral you’re bored of and put the funds back into buying new livestock for the tank. Participate in a live sale (AquaSD and POTO have great sales coming up!), order something off divers den, or just drive around all your LFS till you find something that speaks to you!
