Have played my completionist toon as a druid some time now and he is specced as a caster.
The class is fun and really nice and entertaining now with all the slas. Great job!!
But.. I find the elemental forms underwhelming.
Druids have veru few spells, and because you get -3 in all spells not in your element it be way too much is my experience.
My first suggestion is to make a feat at lvl 19 and 20 who improves the elemental forms. (or if Turbine want to encourage multiclassers make one 17 and one 19 maybe).
And this improved form lessen the - to 1 or get rid of it entirely. And maybe even make the + go up one or two levels.
Suggestion nr2 is to get some more spells for the druid. If you play a pure caster or animal you cant even fill the spell list with spells for that type. It just feel "not finished".
If this is too much time investments, maybe just change the elementalforms as they are to lessen penalty from -3 to -1 and it would be way funnier and more good spell options to use.
Thanks for listening and please give feedback! (especially if you have experience playing a caster druid)