Just bring them back.

why thank you
I tend to think outside of the normal power ranger universe

I think it would be awesome to have them all come back in a powers rangers classic episode, kinda like forever red, off the main time line just a fan appre. kinda ep, that would rock!! all the cool fx applied to the old school rangers...gonna be a hit

[PRangerX]

Forever Red is actually in continuity. But I get what you mean.

[psychic ranger]

That would be great maybe we could get someone that looks like Billy's. Kim's. and Trini's actor/actress and they could do it also do special effects for Zordan and get someone who could do zordan's voice same goes for Ernie the another actor idea not the special effects idea

[MattEmily]

It'd be awesome seeing the originals back but there are a lot of reasons as to why they'll never be able to get every single actor back.

Sure, you don't really need Ernie there. Yes, he's a major recurring character but he was just the Manager and Owner of the Juice Bar and you don't really need to have the Juice Bar there. There's that and the fact that his actor is dead.

All you really need are the Ranger actors. You don't really need Zordon or Alpha however I will comment on this in a later part of this post.
However there is the obvious problem of Thuy Trang being dead in real life. (Rest in Peace Thuy.)
There's also the additional issue of David Yost and Amy not being too fond of their time on the show.

Zordon could be brought back if they wanted to bring him back since Bob Manahan wasn't his only voice. Sure, Bob ended up dying and it would be seen in bad taste if they ended up replacing him but keep in mind that David Fielding did not only his voice but also the head of Zordon for at least the first 40 or so episodes. Also personally I would find it in bad taste to bring back Zordon anyway.

Yes, it was stated by Amit Bhaumik that Alpha 5 has been destroyed but because it was never stated in any episode, it's not officially true.

On top of which because the show is currently non-union, it's doubtful you'll get anyone back from the MMPR days other than Jason, Johnny and Catherine. It's unknown whether Austin, Karan or Steve would ever return to work in a non-union production but it's possible they would.