I think having multiple authoritative DNS servers in different networks might help with this.
I'm not sure how CAs deal with situations when a domain has several authoritative DNS servers, but not all of them respond with expected reply. Would it would be a good idea for CAs to ask *all* authoritative DNS servers and require a coherent answer from them?
> I think having multiple authoritative DNS servers in different networks might help with this.
For the case of the dns-01 challenge type, sure. But if you're using http-01 (or the attacker asks for it), then all those DNS servers will answer back with the same IP address for the A(AAA) query and you're back to the hijacking scenario.