Good grief! It would have been spectacular. I guess you could just look at her endorsements page on Wikipedia and whoever is in that section, probably the most prominent would have performed:
https://en.wikipedia.org/wiki/List_of_Hillary_Clinton_2016_presidential_campaign_non-political_endorsements#Voice_artists_and_musiciansOff the top of my head, Elton John definitely would have performed to spite Trump, especially since Trump is supposedly a huge fan of his.
I wish we lived in this universe instead. I don't want to hear s*** about how inevitability disastrous the midterms would have been. Trump as President is not worth it in any way.