How attached are you and your players to the lore, and how specifically invested is the potential Diedne player in the Dark Secret/Hidden Lineage of it all? If they’re JUST into the Druid Vibes of it all, how do you feel about setting this in an alternate timeline where the Diedne won (and Tremere, or some other house fell), or perhaps where the Schsim war never happened? It might take a decent amount of work to redo the chronology, but oh look there’s already some ideas here.
I do like the Cailleach Magic/Hedge Wizard approach.
If you’re going whole hog Diedne in the canon(ish) setting, and you want Diedne to have more stuff, I’d say pick one (or more) of the Mysteries and let that be their thing. Hermetic Sacrifice seems like an obvious heading to persue. Divination and Augury if you don’t want to have to figure out all the ins and outs of summoning spirits, since no one else has that as their Thing.
Context: A while back I was noodling on the idea of a Diedne Magic character who can use Hermetic Sacrifice to boost their total for Divination and Augury by reading the entrails of the creature so sacrificed.
TLDR, yes, I would let a player be a Diedne, because you’re going to be able to tell interesting stories that way. You might just have to find a way to make everyone else equally exciting, to balance Diedne out. This actually probably works in the character’s favor - if all eyes are on, idk, the Tytalus who’s training a dragon as a mount and the Verditius who accidentally got made into a Bishop, your Diedne can stay under the radar by being less weird.