This was a list I made in 2013.
The Reign of Kindo
Imogen Heap
Hotel of the Laughing Tree
Fish
Andre Matos
Jan Hammer
Eric Johnson
Godpseed You Black Emperor
The River Empires
Pepe Deluxe
Vuvuzela
Faith No More
Arch Matheos/John Arch
Zak Stevens
Gatherer
Archive
Agalloch
Sigur Ros
I can't think of any off the top of my head to add other than Fjokra and Small Leaks Sink Ships for sure. Some others? bands like Bent Knee, iamthemorning, Textures, Spawn of Possession, Major Parkinson, Bruce Peninsula, Neverending White Lights, 22.
And I can remove thankfully Agalloch (2015), Godpseed You Black Emperor and SikTh (2016), and hopefully in a couple of months The Reign of Kindo.
Vuvuzela/Kiss Kiss (but maybe not Josh Benash), Fish, Andre Matos and The River Empires (but not necessarily Jessy Ribordy and possibly other projects of his) all seem pretty unlikely per they seem somewhat retired.
Which leaves among some of those, people like Imogen Heap and maybe John Arch/Arch Matheos being the longest wait.
I suppose I am missing some others like U2 or even Paul McCartney. But that has a lot to do with cost and crowd-size, although I felt the same about Springsteen many years ago, and then was lucky enough to get tickets from my Aunt who ended up with extra tickets that were better than the original ones she got.