What else could I add to my 75? More of the same? Everything is getting along pretty well. I was just wondering if there is one more species to add?
Johannis would be great if you can get 1m/3f. Or P. elongatus would work too. I'd try to add another "semi-aggressive or aggressive" tempered Mbuna not Yellow Labs or Acei which are less aggressive. Kenyi and Auratus are 2 of the most aggressive Mbuna, so its best to get something that would do well with them.
you could definitely get more.
I have seen the number of mbunas you have in a 50 gal before.