Inquisitor as Witch?

I haven't played PoE since after the 2.0 update. With these new ascendancy class things, are you now locked to a particular character? I want to use the power of the inquisitor, but as a witch. If we can't do things like this, it means the game has become more segmented, which would be immensely disappointing.

Anyway, I'm looking into starting a Power Siphon build... would inquisitor definitely be best, or would one of the witch subclasses be viable? (assuming inquisitor isn't accessible from the witch tree)

It is not accessible and is not meant to, although you can access partial power of both classes through scion... Pff.

If u want to build an ele PS then inquisitor is the best, u can either go for elementalist for clear speed. If u want to build an physc wander u can either go for elementalist or occultist. But srsly who would want to use PS over Kinetic Blast .