I am at that stage too, and after reading recommendations and watching review videos, on this website and others, I came to the conclusion that if you want to experiment with a new key, why not try something rather different.
My first (and still only) harp is a Lee Oscar Major Diatonic C, and I think my next will be a Natural Minor Am key (or perhaps Cm or Gm). Why? Because it seems like the range of styles one can play with the Natural Minors fits what I should like to play, and the mentioned keys cover what I think I need.
In brief, it is an experiment. Take it step by step, harp by harp, key by key. And most of all: Have fun!