Bands/artists you never got to see live in their heyday, but would've liked to.

The Paul Butterfield Blues Band w/Mike Bloomfield
Bob Dylan in the Folk City & 'Blonde on Blonde' time frame
Joni w/Jaco
+1 Stones w/Brian Jones 
Yardbirds w/Clapton, w/Beck, w/Beck & Page
Early Fleetwood Mac w/Greenie
Led Zep early on
Allmans w/Duane
Little Walter Jacobs & the Aces