I would love to see other summons introduced. Love the quest for the Black Flame Dancer/Knight and would love to see the other summons you see such as the Wood Golem, Stone Golem and Shadow Specter ...
I like the Summoning Circle idea. So far I've only been able to summon my Wolf Spirit and play with it, but I do think a few other summons that had some dialogue or just interaction when called in non-combat situations would be nice. Intelligent ones could pass on some lore, others could possibly have their own stories.
Would be hilarious if you had one summon that you've been using in combat but, when you finally created your own circle to communicate, he/she tells you that being called to your world (outside of this special Summoning Circle) is hellish, soul-rending pain and that you should never do it again. They should, of course, be incredibly strong summons. After that conversation, there's no warning, but after every 5th summon of them or so you gain Corruption for being an uncaring prick.
Also secretly, after 50 or 100 fights without summoning them, the next time you talk at the Summoning Circle they reward you somehow with some power, item, or achievement. The real secret to it, however, is that it only counts the fights you win where you had the Summon power slotted, thus taking up your Encounter slot, making it a real challenge (though you could farm the newbie Frostwood area to keep it easy).