It's extremely difficult to find a system that can have very low to no fees and that isn't simultaneously open to abuse by a script-kiddie... unless that system doesn't actually store a ledger but rather something like ...balances which shift.
Actually even then you can spam it by creating a lot of balance addresses that would require XX bytes to store, just by sending a no-fee tx of 1 satoshi / duff to a veeeery long list of random addresses. So, I really can't think of how you can make a promise of zero to very low fees and infinite scaling when you are also opening yourself to an attack vector.
But then again, I'm not the coder here.
Difficult, but not impossible.
All these scenarios are being considered.

To be a little less vague, quorum acceptance of dapi requests has some intelligence baked in.
responses to novel attacks are also in the works.