I've mentioned elsewhere about missing seeing Xtc and The Teardrop Explodes, but also high on my list would have been New Musik and Landscape (in their jazzy pre-Tearooms days), & Mansun.
As for realistic, I would like to see The Wild Beasts, & Kraftwerk(missed out on Tate Modern dates last year).
I'm a big Bowie fan too. Only got to see him once, Milton Keynes(1990), Sound & Vision Tour. With Adrian Belew :)
It made up(well almost) for not getting Tin Machine tickets a year earlier, after queuing for about 12+ hours, in Hammersmith! Pre-internet days!!!