yes, i like these and the game needs more like you said to build on what we have.
I will only add, that i think , likely through somelike like the cube, or whatever. Point is when we have said runes activated, a way to also build into these skills could be interesting.
So if sorc has earthquakers from the barbarian, there is a way for them to be enhcanced and altered that is unique to sorcs using earthquakes, and so on for other non-barb classes.
They gain the ability to add things like earthquakes cast twisters but with a sorc these can be frost twisters and frozen earthquakes
Or even more unique the element is customizable, and the new ability is gained so its a frozen earthquake that has a frost hydra heads spawn from inside of it attacking monsters.
On a basic level the element can be changed, but the skill is the same earthquake/hydra in the various elements but will always be a hydra or twister.
Deeper, the earthquake has a variation for each element so fire/hydra quake . lightening, Quakers are electrified and a shoots out ball lighting. So on frost would be something frost related.
Just spit balling here to inspire.
What Op said would be a fun start though. 
Edit- Alternative - do away with cross class. and make runewords and powers class realated and branch it out that way.
So under the hood the sorc has borrowed barb earthquakes with spawning dust devils.
But with sorc its a black hole vortex that pulls monsters in and spawns dark arcana twisters/balls , spears whatever. This way old mechanics are being reskinned and named for the new spells and abilities that give it unique sorc gameplay and visual feel, without creating something from scratch.
Using OP’s Wearbear idea, to prevent all classes being wearbears, with the sorc we get Archon form and get a skill bar of new skills while in that form,
So same concept as OP, but instead of a borrowed power we have new powers per class that is basically a reskin of wearbear underthehood or something like that. Giving each class a form ability that is unqiue to them.
Druids ofc dont miss out, they gain a new form too, as they should have more since they are a druid. This form is based on the rune rules though like it is for the other classes.
Cross class, is not that great in D4 since we can do nothing with it, I think expanding the class fantasy is D4’s stronger suit and this would be one way to do that.