If Aisha had remained she would undoubtfully have become Zeo Ranger II and later on Yellow Turbo Ranger until presumably passing her powers onto Ashley (if it happened as we saw on the series).
Billy meanwhile is harder to say. He stepped down as a Ranger at the start of Zeo, and it took almost the end of the series for Billy to leave. I don't know if he would have become the Gold Ranger instead of Jason. Or maybe once Zeo went to Turbo Billy (who also would have ended up developing said powers) would have become the Blue Turbo Ranger instead of Justin, though I don't know what would have happned when the four veteran Turbo Rangers passed their powers on as was mentioned with the above question about Aisha. Or maybe he would have continued on a technical capacity.