There is a range of strict VST midi plugins out there - Piz Midi plugins to mention one set of a couple of dozen useful stuff for midi.
But these has to be routed like one synth to another, and does not list among midi inserts in Cubase.
If Steinberg made a VST MA(I think midi insert format for Cubase) plugin listed among midi plugins available - that take normal VST with midi in and out - what a boost. This would make manipulating midi a treat in comparison to now.
And you can also add your arpeggiators to normal midi insert part in Cubase.
As most of Piz plugin are gui-less - this wrapper need to support gui-less plugins, as most hosts for plugins do.
Since years I wondered myself that there is no possibility to insert 3rd party midi plugins in the midi channel.
It requires requires some kind of routing to use them. That works but is not smart.