[chasesinister]

Out of the five remaining original cast members I think JDF is probably the most likely to come back.
Amy Jo Johnson has been trying to distance herself from her past as a Power Ranger and David Yost has been vocal about what he felt was discrimination during his time in PR, so those two would probably not come back.
The last I've heard of Austin St. John was that he gained a lot of weight. They may ask him to lose some weight before coming back.
And I don't know what Walter Jones is up to now.
And of course Thuy Trang has passed away. But it would have been really nice to see the original six for the anniversary.