If I could have ONE extra control on any given Tremolo, it would be Level. Sometimes you want a little more juice so you don't lose the sound in the mix when you engage. After that Triangle/Sine vs Square. And then all the rest.
But I have a few two knobbier that I really like; Demeter tremulater, Dano Tuna Melt, Guyatone VT3, and of course, the Diaz Tremodillo.