This is a question that I posted to /r/BitcoinDiscussion a while ago and I'd be curious to hear your thoughts on this too
Cross-Input Signature Aggregation has been something that Bitcoin will likely eventually get. It'll allow you to only provide one signature for a transaction (instead of one per input), making large coinjoins (or even exchanges consolidating UTXOs) cheaper.
But reading about it, it seems that it would require introducing a new output type, as it will be incompatible with the current P2TR output.
The issue I see is that first of, this would introduce a new output type (compromises privacy). Should Taproot have been perhaps delayed until CISA? That way the incentive for upgrading to Taproot would be greater too (huge fee reductions, switching from P2PKH->Taproot-CISA would probably have yielded 70-90% fee reduction for exchanges and other users with large amount of UTXOs), which would be a win for privacy since Taproot addresses can be multisig, or script addresses, or just a simple singlesig, so more users using it would lead to more privacy for everybody.
Also, do you think this is something that maybe a hard-fork could be done over? It seems to me to be the least controversial change to Bitcoin, and doing a hard-fork would allow you to make existing + legacy outputs (Segwit P2WPKH, P2PKH and even Taproot addresses), so the savings would apply to the existing UTXOs.
It'd also avoid another trade-off of a theoretical soft-fork: That spending a CISA input with a soft-fork would likely require you to add a 1 byte place-holder to the witness section (You can't leave witness section empty apparently)
tl;dr: CISA will increase scalability of base layer and increase privacy (by making coinjoins cheaper, to the point it'd be cheaper to join a coinjoin pool than do a transaction on your own). But should it be a hard-fork?
So, assuming the CISA-compatible output had the same structure (which is reasonable) and we could just drop the whole witness stack for all but one input, the average vsize per input would asymptotically converge from 57.5 vB (one input tx) towards 41 vB as inputs grow in count on the transaction. The maximum savings would therefore be up to 28.7% of the input weight of the transaction, outputs and header remain unaffected. It’s not clear to me how that would get anyone 70–90% fee reductions.
1 - 57.5 / 148 = 61%of vsize on the inputs. With full aggregation you could remove the 16 vB (64 bytes) signature from each input. This would save16 / 57.5 = 28%on the inputs compared to Taproot. While the total reduction going from P2PKH to Taproot-full-aggregation-CISA would be1 - (57.5 - 16) / 148 = 72%per input (so lower end of your figure). Of this 72% reduction, most of it,61% / 72% = 85%, comes from going from P2PKH to Taproot with no CISA.