> 'assert that my parent has a scriptpubkey of X'... That way you can, for example, have a UTXO which only allows itself to be absorbed by a transaction also involving a UTXO with a particular capability
I'm not sure I fully follow. I usually think about covenants as having the reverse form, that a parent would assert "my children must have a script of the form XYZ". Are you saying you want to be able to specify that a UTXO can only be spent if the resulting outputs of that transaction all share the same script? I see
this page but i don't understand how those concepts relate to covenants.
Two concepts here. First of all Bitcoin doesn't have a strong single concept of a 'parent', it just has transactions where all the parents lead to all the children. For this sort of trickery to work more information needs to be added to specify which of the inputs is the parent of each of the outputs.
Second what in practice happens is that a coin can check what its own id is, then verify the secure hash chain from its parent to itself so that it knows what the parent looked like. For a Singleton it can then rely on the fact that its ancestors enforced that they each only had one child to know that it's the only descendant. In some sense this is like covenants which point backwards in time although that information is already there in principle because of the secure hash chain but hard to parse.
>
allow references to old blocks so code snippets can be pulled out of them
Nodes currently aren't required to keep around the whole blockchain, but your proposal sounds like it would require them to. I think this could be pretty detrimental to future scalability. Monero, for example, has a situation where its UTXO set is the whole blockchain because you can't generally know what has been spent and what hasn't been. Allowing references to old blocks would pull in all this old block data into the UTXO set. So unless you're very careful about how or when you can reference old blocks, this could cause issues.
Don't full nodes by definition have to have the whole chain? This does make pruned nodes difficult, but it could also have rules like you can only point back so far.