Here's my modular grid page for the Pro 2 companion case:
https://www.modulargrid.net/e/racks/view/783200Right now I feel like it's very much experimental, as I'm new to eurorack and still pretty new to the Pro 2. I go back and forth between if it makes more sense for the eurorack to focus around being a separate voice (or 4) or focus on modulating the Pro 2 via CV and modifying the signal coming out of the Pro 2. Based on what I've got so far, the answer is probably "everything"
Regarding what you said, yes, you can send Osc out via CV. What's interesting is that you can actually set the level to 0 on the Osc and it will still be outputted to CV. I'm guessing the level on the osc config is the level of the mixer going into the filter, and routing the osc to CV out is pre-mixer. In any case, yes it's possible to both use external oscillators and run it through the Pro 2 and to use an external filter, running the Pro 2's osc out via CV then the audio out from the filter back in through the audio in on the Pro 2.