The challenge here is that Van Halen's pop-influenced 1984 is one of the biggest albums of 1984. It's difficult to imagine the band keeping the same lineup and deciding to radically change their sound in one album after basically hitting the jackpot.
So it seems to me that the real option involves Van Halen being willing to turn the page after firing Diamond Dave; a sort of "I don't care how much money we make, I'm not going through that again" mindset. Instead of signing the even-poppier Sammy Hagar, have VH ink a frontman who takes the band in a thrash-metal direction. Ronnie James Dio, perhaps? (Ian Gillan is available, but his vocal chords are shredded by 1984.)
Dio's voice isnt really suited for Thrash. If you want to wait a few years, you may be able to get either John Bush or Joey Belladonna.
It'll never happen with Diamond Dave. Plus recruiting a new vocalist from the burgeoning Thrash genre will be out of the question due to the commercial success of 1984.
Dio would be a good choice but they'd only record in the Heavy Metal genre; not Thrash. Plus, his personality may not gel with Eddie's.
But even if they do find a vocalist suitable for that genre, I have a feeling Michael Anthony may make an earlier departure from VH.
I mean, Dio has done thrash-ish stuff, case in point:
http://www.youtube.com/watch?v=21VECfzsbXc But I don't see him joining Van Halen, as Dio's solo band had actually become successful for him by '86. And his personality definitely wouldn't jive with Eddie's.
As for Ian Gillan, again, he could pull it off, but as has been said, his vocal chords are shot by this point, just listen to him trying to sing songs like
Zero the Hero, or
Heaven and Hell live with Black Sabbath in '83/'84.
That being said, you want a vocalist that can handle thrash, and has a decent rock/metal pedigree by that point, Japanese vocalist Minoru Niihara from Loudness. Case in point:
http://www.youtube.com/watch?v=_ZyVAy41-fI Considering the pop-metal direction Loudness took in the mid '80s in order to break into the American market, and the fact that he would be replaced by an American, Mike Vescera, he's available too.
You want to tie these people together? The first track I linked to,
What Cost War, is written by Jeff Pilson of Dokken fame, who also played bass for Dio. The drummer is none other than the late Munetaka Higuchi of Loudness fame, the same band Minoru Niihara sang for up until being replaced by Mike Vescera for the
Soldier of Fortune, which features keyboardist Claude Schnell, who also played with Dio.