Personally i see it as an add on to JUCE, so follow the licence scheme, flat fee £500 for indies £1000 for companies (Multi seat) if i remember correctly.
Charge again for V2, just make sure it has valid wanted features, same for v3 and so on.
I understand completely that Chris needs to get paid, and that is of course only fair, however, the margin between getting paid and this disappearing and being just a hobby project is very slim, maybe Chris has not been on the scene and seen all of these kinds of developments appear and then just fade away because the sales that were expected never happened and the developer lost interest.
Having a huge fanatical user base that contributes tutorials/scripts/code/UIs and so on, will make HISE a valuable commodity, however, if you charge the ground roots user out of this (Which is what is proposed) you can forget those contributions, because
1 it will be mainly companies and why would they share (They never do)
2 Even if it was some indie developer, they have paid a lot for one app, why should they share their knowledge.
And lets be clear here, there is zero community right now around HISE, pretty much just you and the developer, that really is not good, considering how much of an impact it could have had.
At the prices suggested it is in to mid level territory, you are asking companies who are actually full time to rely on your backend, Kontakt is proven time and time again, so that is the platform these companies are going to go for, there is little to no doubt about that, I work with quite a few, and can tell, you, they have zero interest in moving away from Kontakt right now, their skillset is in Kontakt, their sales are in Kontakt. they trust Kontakt and for something else to even register on their radar, it needs to have been around for a long time and be offering something special.