If you could do trem and phaser together it would get closer to a real univibe. Along those lines – imho if you can add trem to just about any of the other effects it would be great.
As for implementation….
On the tandem effects you could simplify the tweakability of both effects and make the top row of knobs control one effect, the bottom row of knobs control the second, and the X-Knob could control the blend between the two. "Type" could select the primary effect and "Mod Source" could scroll through the menu of secondary. Chorus, Phaser, Flanger, Tremolo would be my personal main choices for those included in secondary.
