The background of this approach is to differentiate the elite classes a bit more from hybrids and, by doing so, give the Lord a new home and make him sort of special compared to the Valkyrie.

Here's the idea:
- Make the Lord an elite class
- Give Ninja and Lord each two spell schools they can learn from (lvl 5+): Wizardry and Alchemy for the Ninja, Divinity and Psionics for the Lord

Of course there are more things you can change (for example I would give the Lord a War Sceptre and a Buckler Shiled as starting equipment and give him Shield as his special skill), but this is basis.

You can also combine the spell schools differently, but for the Ninja Alchemy and Divinity wouldn't really make sense for him (not the most pious kind of guy), and Alchemy and Psionics (which actually would make sense) is too strong of a combination since they have almost no spells in common. The same is true for Wizardry and Divinity, which would also be an overpowered combination - especially on someone like the Lord.

This approach would also get rid of redundant combinations of combat-class and spell school (Valkyre-Lord, Ninja-Ranger).

Do you think this could be balanced?

The intention is good yet I believe both Lord and Valkyrie to be very very powerful anyway. That said I have doubts whether elite xp requirements balance out a new magic school. "Doubts" because I do not really know about the effectiveness in party play while it is almost irrelevant in a solo: the nerf only exists if there are level discrepancies between your characters I'd say. In fact Lords are probably better than Valkyries in usual solos since Cheat Death is a lot less useful.

On the other hand Divine is by far the best school for hybrids and also has very similar spells, so... Psionics is far from "doubling his casting power". It may actually be balanced for group play.
It is probably a small buff to an already powerful class for solos. Then again solos do not need perfect balancing: it is a voluntary challenge anyway so choosing your character's powerlevel, including the very powerful choices, is perfectly fine.