I actually thought Cheryl was at her best last series. Mel started off well and then veered into a ironically-enjoyable troll figure before ultimately becoming a younger, female Louis, barking stock phrases ("Light and shade!", "On point!") and having zero consistency. If one or the other were to come back, I'd have to opt for Cheryl, though obviously a complete overhaul would be nice.
That said, I don't think any combination of judges is really going to draw people to a massive degree, the format is just too tired at this point. The best they can do is look to find the best possible line-up of contestants, choose more relevant/current songs and tone down the obvious, heavy-handed overproduction, which will at least invigorate the show for the audience that's sticking with it. However, given the fact that they actually seem to be courting an older, more conservative audience, I can't see what incentive the producers would have to ditch the relentless ballads and tired old tropes, unfortunately.