Re: ECDSA for allowing a type of proof of work via DER encoding. I think a naive implementation of that could suffer from several gotchas, e.g. see:
I think it could be made safe, but you’d probably want a serious cryptographer to spend a significant amount of time thinking about it before any serious money was committed to any contract depending on that feature.
If there’s significant demand for PoW-based contracts in tapscript, I think it’d be better in almost every way to enable opcodes that allow verifying SHA2-based PoW.