A siabrae has an unholy bond with the blighted earth. It loses any animal companion or access to domains it had from its druidic nature bond ability. In place of nature bond, the siabrae’s close ties to the blighted landscape grant it one of the following cleric domains: Animal, Death, Destruction, Earth, Madness, or Repose. The blightbond ability otherwise functions the same as nature bond.
Would it be too much of a stretch to include subdomains as options for a Siabrae to choose?
Well, druids are already allowed to select subdomains so, I don't see any reason that that would change.
Subdomains can be selected by Druids (except the metal subdomain) and inquisitors (if their deity allows it).