I see that we're still in the holiday spirit, haha! Cheers to a new decade!
Anyway... I had been following Nelsons' performance schedule on his website until recently. It shows his entire calendar on there, so that’s pretty cool. I think Bruckner 5 is the next release in that cycle because I know it's already recorded. They just did the 8th in the past couple months, and they either JUST did 1 and 2 or are about to. I forget, it’s been a while since I’ve looked. As for the Shosty cycle I think 1–3 are due out pretty soon, and I think they recently did Babi-Yar. I'm forgetting if they've already recorded 12 and 14, and if I'm remembering correctly they are doing 15 soon.
I also think a Mahler cycle might be happening soon with Boston. They've done a M5 in the past year, they have that allegedly splendid M2 already on video release, they did a M4 about a month or two ago. They also have a M3 and M1 they performed in the 2017–18 season. Those recordings probably exist somewhere. There's also apparently a live 2015 M6 recording available on Google Music or something:
https://www.bso.org/brands/bso/media/classical-live.aspx/MobileContentPageMy guess is Nelsons is either testing the waters with Boston for a Mahler cycle or they might even just release these older recordings. I'm sure as he wraps up this Shostakovich cycle in the next year or two he will move onto his next big symphonic project. I'd rather he wait another 10 or 20 years to record a fully mature Mahler cycle, but Boston is overdue for some new Mahler recordings
especially because of how great they sound now with their Shostakovich. Not that that directly translates to great Mahler, but maybe you see my point.