Full disclosure, I've only been using Notion for a week and, for the most part, am really enjoying the experience.
I like selecting instruments to place on the stave and it's really nice to have so many instruments included, however this is where my frustration starts. I would like to use some Kontakt VSTs as instruments to improve some of the sounds and add some of the ones that are missing. I've followed the video examples ((441) Notion: Creating VST Templates, part one - YouTube and (441) NOTION 5 ~ Kontakt 5 MULTI VSTi SETUP - YouTube for instance), and have managed to hook up the VST to Notion but wow, is it me or is this a real hack?! Naming the staves twice? Having a "hidden control track"?
There may be obvious reasons for why this isn't possible, but I'd propose an "all in one instrument editor" that allows you to create an instrument that boots up and configures a VST to your preference and that also saves the score and rules settings such that it appears in the instrument list as an instrument. Either by augmenting the current list, or by allowing you to create your own tree (with sub trees) of instruments.
You'd then easily be able to add your custom instruments with a couple of clicks without any faff.