Elder Gazers have Beholder.... Blood Eles have Diseased Blood Eles... Harpies have that ugly greed harpy.... Ice Fiends... nothing I believe that Ice Fiends should have a boss... like an Ice Balron Give him a chance to drop green meat, plat, a tmap... and maybe even a statue But, since the Ice Fiends are easy to kill, spawn 3 at a time, and spawn pretty fast, I'd say make it a little tougher to make it spawn a boss. Have it at like 150 kills to get an Ice Balron to spawn.
There's only about 30 billion mob types that 'could' have a boss variant. Expect more to show up as time is available to develop and test the bosses. I know there are several others in the works just waiting to be implemented and not enough development time free to do so expediently.
I too love the bosses. My dexxer had a lot fun with the lower end ones like that green harpy. That monster was just hard enough to nearly kill me a few times but manageable enough that I've taken it down several times as well. And it was nice to get rewarded with platinum etc which dexxers don't get as often. My dexxer always recalls into moonglow cemetary, covetous harpies, etc to check for low end bosses whenever I'm in between activities. And on my tamer I also appreciate that the REALLY hard ones interrupt otherwise disgustingly lucrative spawns (like EGs, blood eles, ogre lords), forcing me to recruit help from others.