So here's how I'd do it... ^.^;; not that anybody asked or cared...
Start with Zordon (or Alpha or even freshly Good-i-fied Rita... some powerful mentor figure) coming to Earth on death's door, just like you said. They have a warning about a great evil coming towards Earth. Turns out that there's an armada coming that's so powerful that even Dark Specter was afraid of it's reach. So Tommy gets together just everybody... like every character that's ever morphed answers his call. The SPD and Time Force Rangers come in from the future, even, because if these invaders win then there won't be a future and this event isn't supposed to have happened like many of the others (presuming you even need a reason to explain why they didn't come back from the future to help with other invasions).
So the invaders get here and Tommy's assembled pretty much an army of his own. But even with the army they just can't handle the numbers, so they come up with a plan to use all of their powers up as a desperation move, hoping to take the invading force out. It succeeds, but in doing so, it seperates all the rangers from their powers (again) and scatters them off into the reaches of space. But at least the Earth is saved.
... or is it?! Dun dun dun... *couldn't resist*
Most everybody agrees that it's okay if their powers are gone as long as the world is safe, even if the SPD and Time Force guys are stuck in the past now. Hell, a certain pink Time Force ranger would probably find it a bittersweet joy to find herself in the past again. But while he's on his way back to his home planet, Andros discovers that they didn't stop the enemy empire... they just stopped a fleet. He contacts Tommy and agrees to try and find all the powers again, telling everybody to be ready to take up arms again once he gets back. But even just getting the powers, Andros is going to need help, so he gathers up two guys who don't seem to have any love for this empire and they agree to help him.
Cut to some time later and Andros hasn't been heard from in a while. The enemy is getting closer and the Ex-Rangers are getting nervous, then Andros' ship comes back in what seems like just the nick of time. But... Andros isn't on it. Instead, it's 5 new rangers, the leader of which being one of the two guys who had been helping Andros. 5 rangers with the ability to access all of the former power-morphed forms of their predecessors. The good news is that they seem more than willing to fight the bad guys, even if they're doing it for their own reasons and not the protection of Earth. The bad news is that they don't seem especially keen on giving back the powers that they've collected. The bad news for the 5 new rangers is that in order to access the full potential of those powers they need to understand those powers, something that can only be learned from the people to whom the powers originally belonged.
So you've got a substantial camp that's like "Hey, give us our shit back, I'm super serious, man!" because they believe the best way to fight the invaders is by everybody teaming up again. But Andros' protoge is like "Yeah? How did that work out for you guys last time?" and feels that he has a better plan for dealing with the problem.
Further complicating matters is the rumor that the new red ranger was responsible for killing Andros and that's why he has all these powers now. His teammates don't know anything about it, and he's always quiet on the subject. But the line between present and predecessor gets even more divisive when Andros' other protoge shows up also with the ability to access past ranger powers, worse, he seems able to manifest every previous 6th ranger as a acting soldier under his command. He also doesn't seem interested in understanding anybody, he just wants to kill off all the previous rangers to prevent the new rangers from being able to complete their powers again. That could lead to pretty cool scenes like Tommy having to defend people from his Green and White Ranger forms, Doggie Kruger vs Shadow Ranger, etc... and awesome fight scenes are always awesome.
So now you've got 3 factions. The invading armada. The new rangers and the past rangers that trust the new rangers. And then you have the past rangers who don't trust the new rangers. You could even split the line through times, it wouldn't have to be like all the MMPR era guys are in the same camp, or whatever. Also, with regards to any of the rangers whose actors either can't or won't come back you can make them into heroes having gived their lives during the great battle. Assuming, again, that you wanted to address the subject at all.
Eventually Billy and some of the other tech-guys figure out a way to access some of the powers on their own, so Earth will be able to have a ranger that both sides can agree has Earth's interests at heart... but the bad news being that it won't work on anybody who's ever been a ranger before. So they leave it up to Tommy to hand-pick a new 6th ranger. My personal choice would probably be to use Skull - of Bulk and Skull fame - since he's an established Ranger fanboy who's had his own personal journey from bully to vagabond to heroic type. He'd understand the powers because he's interacted with about as many teams as Tommy has and done some courageous stuff too, while the costume would just look silly on Bulk. Of course, if he's too old to be the part ( "I'm may be old, but I can still do it..." ) you could just make up a new guy, I suppose.
Anyways... that's how I would do it... ^.^;;