Music is why I like U2. I don't like U2 for the books, the prints, the T-shirts. They're all fine, but those aren't why I like U2.
Physical versions of download-only releases such as The Eternal Remixes, or Ahimsa. CD, 12", or 7" - but if its a 7", you need more than just a 2 track 7" to justify the subscription fee. Maybe a 10th anniversary, CD version - or coloured vinyl variant 12" - of Wide Awake In Europe. It's the only release that many of us don't have. Secondhand prices of it are simply absurd. It'll fill a gap and still please the trainspotters who have the original.
DVD/BR's to tie in with anniversaries : a 20th anniversary fanclub 2CD+BR of an Elevation show, or a 10th anniversary 2CD+BR of one of the 2011 tourstops with a radically different setlist. And there's hundreds of shows recorded and filmed on the night the band could inevitably choose. Not everything has to be mixed within an inch of its life to have value to us.
How about a 2 x LP of the U23D soundtrack - maybe with "Kite" from the B-side as the last song?
A CD of radio sessions such as promo tour appearances? The band recorded sessions without an audience in 2000 in London and for RAI Uno, which would make a nice release. Same for the 2004 HQ session - https://www.u2gigs.com/show1240.html which could be filled out with acoustic versions from the Atomic Bomb DVD. The band don't seem interested in putting this stuff on box sets, so they could issue them as supplemental fanclub treats.
We know the band have recorded every last noise they've made on stage since around 1991. It wouldn't hurt to let the music out.