I suggested this not too long ago. My presumption is that they'd likely have to impose some restrictions on what sets you can pair with it, like no AR, BR, DP, archery. If that's the case, and since shield lacks a self heal, something like dark or water blast would seem like a natural fit. That being said, since I already have several characters with dark and water blast, I'd probably want to go with something different - maybe ice or elec blast. I haven't gotten into energy blast because of the knockback, but it could make for an interesting pairing as well...