As I said before, their portfolio contains:
FAST Racing series (Racing)
Nano Assault series (Twin Stick shooter)
Art of Balance (physics based puzzler)
Jett Rocket series (platformer)
Personally, if they make a new game in a series or a port or remake of an earlier game, I hope for Nano Assault. They did FAST Racing already. Sports Games and Platformers does the Switch already and I think especially on the platformer side Shin'en isn't that good as others. Art of Balance never was my thing, but the Nano Assault games are quite fun. If they try a new IP, I think they could try a 3rd- or first-person shooter. The Switch don't have much of these and with their speciality fast-paced action, technical impressive visuals and good music they could produce a good experience in that genre. Shin'en has a very small team though, never expect something on a grand scale from them.
The best of those is imo "Art of Balance" + "FAST Racing series".
I hope they do another Art of Balance, i rather enjoyed that.
I also feel like theres plenty of room left for growth and improvements in the Fast racing series.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071