The Suhr SL68 Mk II is kind of a modded plexi in that it has multiple switchable bright caps, PPIM, a switchable 25k / 50k Mid pot, where the 50k switch makes the entire amp tighter and more modern while the 25k Mid pot is more classic and allows you to dial in the signature old plexi "treble crash" sound that makes the amp almost go into like a fuzzy Q-tron impression when you max the treble and hit the input hard, and a phat switch that changes the low end filtering. No extra gain stages or anything more extreme like that though.
So if a "slightly feature-extended" plexi counts, I'd say the Suhr SL68 MkII.
If we're talking modded to the moon and back, then probably something like a Bogner Helios 100 like @NorCal_Val mentioned which is basically what happens when you take a Plexi foundation a mod it into being a modern high gainer.