I would say Hyrule Warriors....but Dragon Quest Heroes 1+2 apparently has differences between other versions (which might affect enemy density due to processing), that i wouldn't be surprised if a HW port on the Switch took 3DS content with slightly better graphics or something. if it's a new game built from the ground up, then i wouldn't think that Fire Emblem Warriors would suffer from problems. a Hyrule Warriors 2 on the other hand, i could see happening since they have more games worth of material.
Always the Bayonetta games. though it might be seen as promoting games that they've already released. though if both games were on one cartridge, they're relatively new enougth to justify the price. unlike a majority of ported-over Nindies. guess there's also Pokken if they're not going to update the Wii U version. Smash feels like it'd have a 4.5, the cost for additional content in that game felt like too big a hurdle for actual playability. with Mario Kart 8, you were getting a handful of characters and 16 courses for ?14 and the online was much more acessible, compare that to Smash's costs for borrowing the IPs....and a severe amount of Mii costumes (when Nintendo themselves are downplaying them. seemingly only keeping them to a minimal exposure because Mario Kart 8 used them) that you couldn't use online except against friends.
The elephant in the room would be Star Fox Zero, just because it was one of the last major Nintendo titles to even use the Gamepad (to the extent that BOTW shown it during E3 once then dropped it), it'd be kinda hard to even emulate aspects like having characters talking to you as if you're in a cockpit without something like a devoted phone app or something. a separate Joycon mode might be the equivalent of using 2 Wiimotes. also Zooming in things might be problematic unless its like older games.
Nintendo could remedy slightly older games by releasing them as bundled remasters or something. even with Star Fox Zero being a reboot, i probably wouldn't mind if Star Fox 64 3D was bundled with it. or having the Zero assets re-purposed for a Star Fox (original) mode. the series definitely needs a more decent high note before inevitably being remade again for the 3rd (or so) time...not even counting games that presumably happened after 64.