Famitsu have just published an interview with the Sega Ages team regarding the recent twitter survey of most requested games. If you don't fancy reading through the rather confusing google auto-translation, then
nintendolife seem to have the best English-language summary.
The most important take-away is that the team seem to have gotten Dreamcast emulation running on the Switch to some extent, although they seem a little vague as to whether they would use this approach or porting to bring Dreamcast games over. They already stated earlier in the year that they are keen on potentially bringing over Saturn and/or Dreamcast games to the Ages line-up, but have already confirmed that they won't be included in the initial batch of 15 games (which will consist of arcade, Master System and Megadrive/Genesis games only).
Despite Jet Set Radio topping the survey of most requested games, they appeared to pour cold water on any chance of it coming to the Switch any time soon. Although this seem like bad news, I have a (largely baseless) feeling that it could be because there's some kind of multi-platform port of Future (or even a brand new game) in the works.
Lastly, they also mention wanting to do the arcade Super Monaco GP at some point in the future. That was a title, much like Virtua Racing, that seemed destined for the 3DS classics, but never quite made the cut. Since Virtua Racing made it this time, I'm hopeful for Super Monaco.
Just a reminder of where we are right now:
Sonic the HedgehogMega Drive 20 September 2018
Thunder Force IV Mega Drive 20 September 2018
Phantasy Star Master System TBA
Alex Kidd in Miracle World Master System TBA
Gain Ground Arcade TBA
Space Harrier Arcade TBA
Out Run Arcade TBA
Columns II Arcade TBA
Thunder Force AC Arcade TBA
Sonic the Hedgehog 2 Mega Drive TBA
Virtua Racing Arcade TBA
Of the 4 as yet unannounced first batch games, I would guess that Streets of Rage will be one, and maybe an arcade Shinobi game will be another (hopefully Shadow Dancer, but probably the original). If I was feeling crazily optimistic, then I would say Golden Axe: Revenge of Death Adder would also be one, but Sega has inexplicable forgotten about it, despite Golden Axe being one of their best known franchises.