What would Lupin see in the Mirror of Erised?

There is no canon information on what Lupin would see.

In the books nor the films does Lupin ever look into the Mirror of Erised.

I've also searched Accio Quote and Wizarding World. The only thing remotely related to Lupin's desires, is the following quote by J.K. Rowling from a 2003 interview by Stephen Fry at the Royal Albert Hall:

[...] Lupin’s failing is he likes to be liked. That’s where he slips up – he’s been disliked so often he’s always pleased to have friends so cuts them an awful lot of slack.

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But let's not forget, the Mirror of Erised is not a cloakroom mirror, just standing somewhere. It's a unique magical item, and not entirely harmless. As Dumbledore tells Harry:

"Men have wasted away before it, not knowing if what they have seen is real, or even possible."

So it's not likely that many people have been able to take look, Lupin being among the many who have never even laid eyes upon it.