Another tour celebrating SFAM: Good grief, no.
Another tour with a different setlist: Are we talking about after all legs of the DOT tour are over? Or what they should do after the conclusion of the first European leg? I think the band has clearly indicated that there will be another North American leg and (I think) a European leg where they do SFAM. I think they should do both of those things, and I think the NA leg should focus on DOT, with other stuff from throughout the catalog. As far as what they do after all legs of the DOT tour are over, I think they should take their customary break and get to work on the next album.
D/T Tour live album: No.
Immediately start working on DT15: See above.
Another concept album: If the next album turns out to be a concept album, that would be cool, but I don't feel like they should set out with that specific intent.
Side projects/collaborations: With one exception, I have never had any interest in any DT side projects/collaborations/solo projects. The one exception was LTE, and I didn't get into that until well after the fact.