This is more like a business challenge than a tech challenge 

Any Persona game, 3D Dot Game Heroes, Fallout 4 or New Vegas, Tomb Raider games, Skies of Arcadia, Maniac Mansion Day of the Tentacle remaster. Most of these aren't ambitious but I still like to see them.

the Batman Arkham series

The Arkham series (although Arkham Knight is the only one on a console beyond Wii U level of capability, so it's the only one that seems worthy of PB's time), Devil May Cry 5, and any of the latest Resident Evil hill games without any of that cloud BS.

Master Chief Collection

A modern COD game Id love to see ported

Personally, I'd love to see them bring over Titanfall 2.

I hear really good things about that game's campaign, and now that the engine and presumably a lot of its code is already up and running on Switch via Apex Legends, plus the fact it's a 60fps game on base 8th gen consoles, I'm sure Panic Button could pull off an excellent conversion.

My pick wouldn't make sense at this point but I'd love to see them take a stab at an ARK switch port. No doubt it would be better than the current one but I'd be interested to see how playable they could make it.

I’m thinking a bit selfishly and I hope they can be involved in a Soul Calibur VI port. I’ve talked about my hopes of playing the game on Switch to death so yeah... Will it happen? Not likely. Harada doesn’t seem to care about the Switch sales, especially in Japan.

I guess Halo would be cool since Panic Button is a Western developer and they’ve ported mostly Western games on Switch.

Tomb Raiders series.
Nier Automata