Who I have seen is pretty amazing & wonderful. And there are some I didn't see and don't care, most notably Zeppelin, Bowie, Pink Floyd, and, I suppose, Queen (who apparently qualify as a big deal).
Friends have convinced me in the last decade that my indifference to McCartney was an error by saying, "It's the closest you'll ever get to seeing/hearing the Beatles live," which, when considered, has to be correct.
Note: I was just a bit too late for Hendrix, Doors, Joplin, and anyone else from or close to that class of early exits. And, I obeyed my parents when they told me I was too young & couldn't go to Woodstock the summer before I got a driver's license.
I saw ABB's first post-Duane tour, and my first Dead concert during Pigpen's hiatus though he was still alive. This all reminds me that my first full opportunity to see Clapton would have been his "461 Ocean Boulevard" comeback/reboot. A ticket to anything past Derek & the Dominoes (even w/o Allman) has never much interested me. Missing Watkins Glen is totally on me, so no Band either (oops).
Here's my answer as to lost opportunities, including who remains a possibility:
LARGE-SCALE ARENA/STADIUM
U2
Peter Gabriel
REM (though I moved from New Haven 6 weeks before they played Ron's Place and I no doubt would have been there)
MEDIUM SCALE
Aimee Mann
Bryan Ferry
Ry Cooder
Replacements
Husker Du
Pretenders
There must be somebody missing from this list who I still could see, and would want to.
Bob Mould and Chrissie Hynde definitely qualify as available solos. Ian Hunter too, even more than w/ Mott. Sadly, not Adam Schlesinger, though he fits with another list below.
TOO-BRIEF MOMENTS IN TIME, SO I CAN'T HOLD IT AGAINST MYSELF
Sex Pistols
Television
The Las
Stone Roses
ANYBODY I FIRST HEARD OVER 40, OR WAS INTRODUCED TO BY CHILDREN I LIVED WITH DOESN'T QUALIFY, BUT STILL...
Nirvana
Radiohead
Fountains of Wayne
Radiohead
The Strokes
Arcade Fire
White Stripes
Death Cab for Cutie
The National
Beastie Boys
I was the chaperoning/driving parent for Weezer (w/Dashboard Confessional) at the Meadows in Htfd one year, and bought a scalped ticket. I sat separately (in a better seat at lower cost), rather than waiting for hours in the parking lot.
Contextually, as to bands listed just above, it's weird that my point of entry to Dropkick Murphys, Luscious Jackson, Pinegrove, Sunflower Bean, and some other performers is through knowing parents of key members.
All things considered, I think this turns out to be more of a gratitude meditation than a list of regrets.