Weak powers that upgrade over time are my jam, as long as everyone improves at basically the same rate. 

And if you don't mind my two cents [@Dragonfly 9], I think a demon that grants a load of powers to a host is likely beyond the scale of what Whale is going for. It sounds like the plan is extremely weak single-concept powers as the magic at the start. Maybe you could maybe the characters magic be demon binding, and they accidentally bind a demon to themselves and lack the ability to undo it, but the demon's powers are also hindered by a human form that isn't used to magic. And then as they improve, they get to use that demon magic, which could maybe be in one or two of the directions you mentioned maybe? Just an idea, and I am just guessing mostly, just had some thoughts since I do enjoy a good two characters in one body gimmick.

Also looks like we have 5 interested people now. That went really fast.