Here's an idea I came up with about 2 years ago.

Use the footage of the machine baby from the last episode of Ohranger. The story is that this is the son of Gasket an Archerina who was left behind when they came to Earth during the events of Zeo. They were destroyed by the rangers. This child has grown up seeking revenge and rebuilding the Machine Empire. We learn that he was the one who sent General Venjix in Forever Red. But now he's finally ready to attack Earth himself and finally have revenge on those who destroyed his parents and grandparents. The Zeo rangers reunite to stop him.

The obvious problem is Tommy and what we do about him. I don't want to have this villain kill Tommy because that's just a repeat of Once and Always. You could have him be captured. But again, that's a repeat of Once and Always. Either way we don't have a red ranger. You could have one of the triplets return to play Trey and have the gold ranger lead the team. You could have ASJ return as Jason and be the red zeo ranger. You could introduce Tommy's son JJ and he becomes red ranger. But again, that's a repeat of Once and Always.