I’ve asked for this before too, would be great.
I use a few different hardware synths, and even if they have arpeggiators they all might work differently, so I end up having to program the arpeggiated tracks on Cubasis if I want consistency. Having a MIDI FX Arpeggiator would make it so much more simple, and it would add an Arpeggiator function to synths (hardware or software) without one.
Other MIDI effects would be great too, like a MIDI Delay, or even some routing tools.
It would be great as well to have a way to combine different Mono Synths in order to make a Poly Synth, as MIDIPal does.