So over the course of the series, Steff has done plenty of songs with Ash and sung Shut Up and Dance by WALK THE MOON with Gunter at one point. And I began thinking:
What songs would she perform with the rest of the Moon Theatre Troupe if she had the chance?
Normally Rosita saves her duets for when performing with Gunter. But if she made an exception for Steff, I think Girls Just Wanna Have Fun by Cyndi Lauper would be a fun one for them. Or Good Time by Owl City and Carly Rae Jepsen, given that Rosita's song choices are mainly by modern pop singers.
After that soulful rendition of Stevie Wonder's Don't You Worry 'Bout A Thing Meena did, she and Steff could probably both sing another '70s classic. I'd say Knock On Wood by Amii Stewart.
I always envisioned Johnny singing Everything I Do (I Do It for You) with Steff. Specifically the rendition sung by Boyce Avenue and Connie Talbot so that the piano would be the focal instrument.
Given her big Could Have Been Me number, Porsha seems to have an affinity for hard/glam rock. So if Ash and Steff asked her to perform with them, something by Heart could be fitting for their vocal range and harmony, like If Looks Could Kill or Never.
In some alternate universe where this rodent wasn't a total asshat and would be decent enough to share the spotlight... maybe It's A Good Day (from Seth MacFarlane's album with Liz Gillies) would be great for him and Steff. Or maybe even Jazzman by Carole King.