I'm going to work off the assumption that "mainspec before offspec" will remain. The thing to change is how we define main and off.
This may vary with the server or individual, so please keep in mind that this is just my own experience. However for purposes of being better than everyone else, I will assume that my experience is the right one and everyone else is doing it wrong.
What is a main spec?
In a PUG, it is the spec you came as, with the exception of respeccing specifically for the run.
In an organized group, either with a guild or friends, it is your most common spec. It may be a spec that defines your character.
What is an off spec?
Anything other than the main spec.
I propose that a new spec type will come to exist: secondary spec.
Main spec will remain the most common spec, a sort of RP-based identity, what you are most comfortable playing or that your guild expects you to play.
The secondary spec will be the alternate spec. You'll play it and you can become it in five seconds, but it's not your main. It will be a middle ground between the current main and off; closer to main, but not quite.
Offspec will be the third (or fourth, fifth, sixth... hybrids need decimal specs). It is not something that you can easily switch to, having the cost you see now when respeccing, but it will be more unusual than it is now since 5 second and free will be the new standard for role-switching.
Secondary specs may eliminate the current exception on mainspec before offspec: respeccing for the group. Now people are unlikely to fully respec and so it's hard to really say to outsiders what is main. I think PUG loot is going to start going by current spec without "I respecced for you, let me roll like my main spec."
In guilds, main specs will go first, then secondary, then offspec. If I am prot/holy and you're prot/ret, I win on holy gear, you win on ret, we are equal on prot.