I was actually going to suggest something similar: limiting the number of identical grimoires per file.
The idea is that you can have 3*unit, 3*character, but only 1*grimoire.
This would heavily reduce the power gap (<insert Luckysvn777's "it's experience!" rhethoric here>) of vets against newbies, by:
1) Reducing the difference in SP generation.
The newbie has at most 1 SP-generating grimoire of his sphere, the vet has 3 and a Proxy that allows him to use 3 more from another Sphere (usually Meaning of Failure, for obvious reasons).
That's SP+1 vs SP+9~12. With such a power gap, the game is usually decided before it plays out.
2) Limiting the impact of Return-spam.
Water Emperor+3 Returns+3 Cyclones... what's the newbie going to do? He barely has units/chars over lv3, let alone 3 copies (ok, the Gowen newbie has 3 Devouring Lizars. Good luck taking out Water Emperor and its cohort with that), and since he didn't play Proxy+up to 6 of a combination of MoF/Spirit Pact/Rapid Growth/Blessing Powder, he can't really field high-level units, anyway. At least not before it's too late.
3) Limiting the number of revivals.
Strong units are nice, and some of them (Mercenary, Guardian,...) have low rarities, so are availlable to newbies, with a bit of luck.
But you know what's nicer? Multiple copies of them.
It's disenhearting enough to know the huge efforts you needed to deploy to kill that ONE copy of <insert strong unit> are going to be needed twice more, but it's even worse when you see that Instant Revival/Over Soul/Parts Recycle/other reviving trick.
There's just no way for most starts to defeat 6 copies of most of the things fielded against them (us).
One copy? If the newbie is smart, s/he'll find a way.
Two copies? Hard but doable.
Three copies? It's going to require most of the file, a bit of luck and probably a mistake from the vet.
Four copies? REALLY hard, but not completely impossible.
Five copies? It requires a miracle. I think the only time I managed to kill 5 copies of a strong card was someone's Afel (Refess, don't have any damaging grimoires. He'd basically wipe my whole field before I could do anything)
SIx copies? Downright impossible.
As a major side-effect, we might start seeing some variety. Meaning of Failure could get 20 more alternate arts, all players would've seen all of them by lv16. That's how common the thing is.