Currently the only option is to monitor an interface (the various external services will suffer the same problem with even less means of control), but with IPv6 and its multiple IP addresses this doesn't always return the most desirable address out of those available. So it would be great if there were some options to guide it. Most likely one will only want the GUA, anyway, but there also could be another GUA from the privacy extensions, plus there might be reasons to pick an ULA (I don't see a use case for that but who knows, maybe one could use DDNS for internal use inside the DMZ or something to avoid rolling an own DNS service? People can get pretty creative :) ), and maybe even to allow use of deprecated addresses.
Anyway, the issue is that with IPv6 there are more than one address per interface, that also appear in a random order (most often the GUAs are on top, with the non-PE one first, but occasionally the ULAs will be, or even the LLA, so one can't rely on this ordering at all), and thus there needs to be a means of control over which one gets picked. For example, on my WAN IF, I have (at minimum) five IPv6 addresses (1xLLA, 2xGUA(1 PEA), 2xULA(1 PEA)), plus two more deprecated PEAs (again, one GUA and one ULA) until they fully expire. If one could use a Dynamic IPv6 host alias as source this would reduce the ambiguity to 1 GUA and 1 ULA, and I suppose ULAs won't get picked anyway, so it would be unambiguous, but it doesn't allow the selection of this type of alias.
Thanks for considering! :)