That just invites a sybil attack. Large holders will just split their stakes into more shards.
A somewhat radical idea:
One of the issues with PoS is that stakers get rewarded twice: they get the block rewards, plus they get all the upside from increases in the token price. There’s a feedback loop.
To break that feedback loop, one (weird) possibility would be to have the staking token be a wrapped “stablecoin”. In some sense, this is analogous to how PoW mining works: miners pay in their local fiatcoin for ASICs and energy, and get ZEC rewards; their mining hardware depreciates (and by analogy, if we used a fiat-pegged currency for the staking token it would suffer from inflation). They can choose to sell the ZEC they mine to reinvest in their mining op, or they can HODL for hard-money gains, but they don’t get to double-dip.
It’s an open question though whether PoS would actually be viable under this sort of scheme; I haven’t done any analysis to see how it would affect PoS’s security assumptions. And, of course, the idea that one might stake not-ZEC to secure ZEC is just a weird idea to begin with. But it, or something adjacent that breaks the feedback loop in the same manner, just might work.