Top 10 gimmicks wrestlers had before that would work now in the WWE
 

Jericho's Loophole gimmick, the nWo?, Crow Sting? Maybe DDP's self high five "i only work with me, myself, and Page" type shit?

bring da noise

Million Dollar Man/Corporation (the concept, not necessarily the actual wrestlers in the Corp)
The Heenan Family (see Million Dollar Corp)
Jake "The Snake" Roberts
Macho Man
Mr. Perfect
1-2-3 Kid
Lex Luger's steel forearm
The Spanish Announce Table
Hassan/La Resistance/Yokozuna
Submission holds that actually end matches

If you mean good gimmicks that were underexposed in their day and therefore could be given another shot today without inevitable comparisons (rather than main event gimmicks being reused):

Waylon Mercy
Red Rooster (the original 'I can turn anyone into a star' gimmick, not the eventual 'Terry Taylor now believes he's a chicken' gimmick)
"Devil's Advocate" (Sean O' Haire)
Se7en (would make a good foe for the Undertaker)
"The Redneck" Dave Schultz
Bad News Brown... oh wait... they got Ezekiel, right?

I think that Mercy only could really work with Dave Spivey I mean all he was, was a southern Con man. It was Dave that gave the character it's appeal. Now when you say Se7en do you mean the "Child Abductor"ish Dustin Rhodes was gonna do in WCW or Raven's gimmick he was gonna do?

I'm tempted to say the Loose Cannon gimmick of Brian Pillman could have really gotten over now and a lot better than it did when it was done.

And possibly the black hart gimmick of Owen's

Shawn Michaels' cocky heel 98

Tugboat

If it were not done already, something like the nWo with John Cena joining some top heels would be pretty fucking epic.