A recent tegra would be a start.
Well there is only one new tegra that they could feasably use right now, the x2, which isn't that much stronger, though it would probably be enough to smooth out performance by increasing framerate or resulution in games which the switch can't perfectly handle (xenoblade 2 portable mode cough). Though the most significant upgrade would be battery life.
I'm mainly worried about a New 3DS-style revision which has exclusive games, because it locked me out of getting games that I wanted to get for the console. As long as I don't need to upgrade, I'd be fine with anything really.
That kind of upgrade won't be here soon, at least not untill late 2019. It took them 4 years for the N3DS. Also took them 4 years for the DSi.
And besides the brunt of the games would be compatable with the original system unless nintendo goes with the continuous upgrade model that smartphones do. Even the GBC, which was arguably a new generation, didn't have that many exclusives.