You really only have 1 option to create more space for corals since you have a very minamilist setup and thats to add more rock. 1 thing that could work good for creating alot more area for corals would be to build up some rock to the height of the arch a little ways away from the arch, then get a nice size flat skinny rock and use it to connect the 2 features the new rock pile and the arch together giving you a nice size area for more corals especially sps since it will be towards the top of the tank.
This is the way I setup my scape, Alot of live rock stacked up and a bunch of those flat skinny rock. Doing this gave me a huge surface area for corals and you get the added benefit of a much larger and stronger bio filter and alot of areas in the rock for the fish to sleep or just cruise through it and somewhere they can just chill. Heres a pick of my tank which is on the opposite end of tank scapes compared to yours.