Jack Kemp, Lamar Alexander, Orrin Hatch, Gary Bauer, Steve Forbes, possibly add in John Danforth, Frank Keating...
Jack Kemp, if he ran would have an advantage in the primary as the established next-in-line. Though outside of Keating and the perennial Forbes, the GOP still suffers from their generation gap that GWB bridges IOTL. Of all, I'd say only Keating would present as strong of a challenge against Gore as Bush did.
Really odd, actually, how very few Boomers were set up for 2000. There were plenty out there, Bill Frist for one, but were too new to have the type of establishment support necessary to win a GOP primary.
If Gore wins in 2000, GOP wins in 2004, possibly with Bush/Giuliani/McCain. If not, then that means that Congress and the State Houses are so heavily GOP after 16 years of Dems in the White House, that butterfly in a whole host of GOP contenders beyond the usual names. Romney could still be a factor, likely serving a second term in MA as of '08.