I am not familiar with the specifics of signature aggregation, but I have read that in some cases the size of an aggregated signature containing N signatures still has the size of one signature. I don’t know it that would be possible for the signatures of the validators, but it could safe space.
We could use a scheme similar to the “Partial validation and slashing condition” scheme in this post, to do the do the verifying of the signatures off-chain. If a whistleblower misses a validators signature, it can proof it on-chain, burn half the validator’s deposit and keep the other half.