I could see them continuing as a 5 piece. If the band were to write an album after a hiatus first, upon touring they'd have new material to play and as already pointed out by Hahninator, there's plenty of songs Mike could take lead with - LOATR, Iridescent, SOTD, maybe Final Masquerade. I realise I'm stating ballads but that's likely where Mike's range sits.
The sets wouldn't need to be so long either - play shorter shows in smaller venues. I saw Evanescence in June, in London - they played 18/19 songs, and that didn't feel overly short for arguably such a big band.
Whatever they choose to do, the fan base will rally behind them, just look at the outpouring of support after Chester's passing.