It's an interesting idea. I'm not sure that all the combinations make sense. Excluding the connected output script or value but still signing the prev tx hash appears pointless: the script cannot change anyway, and you still need to know what it is to actually calculate the inputs to it, so what is the point of this?

I also worry that quite a few of these combinations could be unexpectedly dangerous. If you don't sign the prevout hash or value and combine it with a regular pay-to-address output then you've effectively written a blank cheque that can be used by anyone, to claim any money ever sent to that address ... no? And then any p2p node or miner could do so, making the transaction pretty useless.

That isn't inherently a problem as long as people understand which combinations have what effects or cannot be used for various reasons. But it would need good documentation and careful thought to explore each possibility people might use.

I'll leave the soft fork business to one side for now. I think any change in CHECKSIG or new version of it would likely be ready around the same time as the hard fork we need for changing the block size limit anyway, and it's much cleaner to do it that way.

The most important change that we need in sighash calculation, IMO, is ensuring that you don't have to hash data over and over again without a good reason. The current sighash definition is unfortunate because it's possible to make small transactions that involve hashing huge amounts of data. It's not clear to me that your proposal fixes that: ideally there would be one exactly one sighash for one transaction no matter how many checksigs are involved in verifying it.

thanks,

-mike