For me, ideally, the new DAW - whatever it turns out to be - would have all the basic SONAR functionality (shouldn't be a problem, given it's what BandLab paid for), possibly including some changes to facilitate whatever direction BandLab wants to take it in the future.
I wouldn't be surprised if there are some outstanding code changes by the last team that didn't ship before Gibson pulled the plug, so we may get a couple fixes or whatever. I'm not expecting too much to change, given the timeline.
In terms of plugins and whatnot, I'm hoping all the old Cakewalk stuff (especially ProChannel modules) continues to work. Luckily, I ended up upgrading most of the stuff I really cared about (TH3, Melodyne, Rematrix, BReverb) to full versions, so it's just the Cakewalk stuff that's up in the air.
Blue Tubes? Can't say I've ever really found much use for those plugins. I'm fine if they stay, won't cry if they don't (besides, I'm not sure if they're in that much danger of going away, if you already have them).
35mm
It's also tricky to work out what they could do about making it cross-platform which would ultimately be an advantage to the company - Bandlab. It would take a lot of investment, but maybe now would be the time to make that investment and perhaps release a multi-platform version next year. That could then change Sonar completely as to maintain a cross-platform product would require some changes to the code base. Although, that wouldn't be noticeable to us.
The good news on that front is that - while not a completely solved problem - there's been a lot of work done in the area of cross-platform code over the past several years. Without a detailed knowledge of the codebase, I really couldn't say how much work it would take to make a post-SONAR DAW cross-platform, but I'm guessing it's pretty doable if there's a will to do it. Also a good reason to refactor the code to allow maximum sharing between versions, which would doubtless be of great benefit.