Has there been a consideration of signing the actual ring and commitments instead of the key offsets? (This change would be a hardfork I think).

The reason I am asking is that we discovered that we can't currently do any kind of protocol on Monero that requires collaborative pre-signing of transactions. With pre-signing I mean signing a transaction that spends an output that hasn't been included yet.

Due key-offsets being part of the transaction prefix, I would have to "guess" what the key-offset for my output is in order to produce the same signature hash as a verifier.

One way to get around this would be to hash the actual ring members and commitments instead of the key-offsets. In order to not bloat transactions unnecessarily, we can still include key-offsets for the verifier to look up the ring members. They would just have to be part of the transaction that is not signed. That shouldn't cause any trouble though because if anyone tries to mess with the offsets, the signatures won't verify as the actual ring members are committed to in the signature hash.