Regarding price drops at E3, here's what I think Microsoft, Nintendo and Sony should do and what I think they will actually do -
1 - Microsoft
2 - Nintendo
3 - Sony
Wii U is in most need of a pricedrop. But as you said, if they're selling a Splatoon bundle for 300$ I guess we won't see any announcements about a price drop at E3. It's possible we will see one in the fall, though. It all depends what Nintendo makes out of every unit sold. They're not going to drop the price and lose money.
I really hope Microsoft will unveil a slim Xbox One. I really do. But at the same time, we should've heard something by now since virtually nothing is kept secret anymore. And in any event, Xbox One will do fine this holiday with Halo 5, Rise of the Tomb Raider and strong multiplats.
As for Sony, they don't need to drop the price yet. Microsoft were able to beat them in NA in April by a nose. I wouldn't be too concerned if I were Sony. I think they are working on attractive bundles for the holidays instead of a price drop.