The REAL question here is, should the pacman 3.0 release be delayed for something like this, or should we hold of until 3.1?
Such a change like that (new backends) would illicit a major version increment I would think. Nothing says there must be a huge lag from pacman 3.x to pacman 4.x. ;) In regard to the backend talk. I think the number of possible backends should be very limited. Think of it from a supportability standpoint. With people using different backends, when something happens with one users pacman install, it would be much harder to pin down the issue. More backends means less users running the same basic configuration, harder to support, etc. I say choose a single backend. Modify it as needed, but keep the backend choice simple. Ie... one backend.