Do you enjoy the behaviour of the mbuna you have?
If so, you might find fronts or oscars a bit "slow" and not very exciting. (I did...)
It took me a very long time to get to where I was happy devoting a tank to the slower moving species.
Oscars are quite messy, but very personable. I'm just not sure I'd want to give a couple of them that entire tank!
Fronts are beautiful, but they have a totally different demeanor than mbuna.
Have you considered turning it into a Tanganyikan "community" tank???