Kenpachi's also a good candidate since I don't believe he's been seen yet
I think the sword in-panel with Urahara was too clean-looking to be Kenpachi's; isn't his supposed to look beat up? For what it's worth, Grimmjow was the only person I could come up with that would fit both Urahara's reaction and the power to beat the Quincy, I don't think Aizen would be likely just because it's too soon and it seems like it's the Hollows' time right now.