Okay... for just a few minutes, imagine something midway between a fireball and a magic missile.
Something like, oh... say...
Grease... A spell that gives you the option of knocking someone prone or making them drop whatever their holding.
Imagine something that's a little too powerful to used at-will, but not quite powerful enough to be an encounter only power... I'm not proposing making Encounter powers rechargeable, I'm proposing adding a "rechargeable" category in between the two for PCs, to fill in that big quantum gap that lies between them.