I like listening to early demos of tracks (Kindergarten Achtung Baby) and even sound checks because I love the raw form. So demo albums would be a great win for me. I think u2.com subscribers or people thinking of subscribing aren't exactly your casual fan (especially in quiet non touring years) so risks can be taken! If Bonos voice doesn't sound great or The Edge's guitar is off, we won't care, we'll love it!
Another good gift idea, which other people have alluded to would be a high quality picture book (like that really expensive product design book Apple released a few years ago), with images from past tours, big moments, propaganda covers, etc...
Main thing for me with the gift is I want exclusives! Berlin (well half of Berlin) was on TV, I loved it but boy that was a cop out.
Away from the gift, with content on the site, I don't think there's much to entice new subscribers or keep subscribers happy - especially when there is no tour, its very quiet. The suggestion of full live song footage is great, the odd minute snippets are okay for free view site users but give subscribers the full ones! I just think with a band that's been around for as long as U2, there's not enough valuable content!
The watch-alongs were okay but they were just streams of DVD releases, which I'm sure most of us already have or have at least seen, give us new stuff!
P.S. I'm actually not too picky with the gift, just no remixes, please, I beg. Also last years gift - a DVD...in 2020...still upset it wasn't a 4K blu-ray let alone a standard blu-ray!