Give conjurers an elemental path to follow. Add more familiars, more demons/angels, devils/archons such that they fit the theme of that path. In other words earth conjurers might call up a woodland creature while water conjurers get water or amphibious familiars.

Also, make familiars similar to shifter forms - i.e. you get a different one to call at certain level points until you have about 3-4 different familiars, each with potential pros and cons.

What I haven't figured out is to how to keep this balanced. It's nice to get a fairly powerful familiar early on to bridge those 5 levels until you can safely call elementals, and once you get up to calling demons/devils, conjurers don't really need any additional power, just some flair. The only idea around this I can think of is having the higher form familiars incompatible with something else, say elementals