I'm thinking of a Witch/Shaman class that can use a mix of arcane and divine spells. I don't what the player to do that using multi-classing.
I imagine that it's gotta be more than just "can cast Cleric and Wizard spells" since
that would be pretty easy to do through multi-classing.
I've definitely seen some decent Shaman-style homebrew classes on various online sources, but for the most part they're
not Cleric/Wizard spell casters... in fact, a lot of them seem to be attempts to pull in other edition (or even other game like Pathfinder) Shaman classes that are mechanically very similar to Warlocks...?
But, yeah, just start thinking at a high level around
what you want this class to be able to do. Think of some cool features and then start thinking hard about how powerful each of those features is, how they compare to features offered to other classes at similar levels to make sure they align somewhat well, think about
how the class casts spells, etc.
www.reddit.com/r/UnearthedArcana/comments/73sfp1/class_design_101_a_fundamental_guide_to_5th
This is a great read on how to approach homebrew class design, and it even comes with some useful Homebrewery templates at the end for null casters, full casters, half casters, third casters, and pact casters to give you a nice outline around the feature structure.
Apologies in advance if we're not allowed to post Reddit URLs in here.