While I agree with you on Annie and Enya (probably my most favorite singer ever), I don’t think any of the ones you’ve mentioned have anywhere near the star-power that Tyler and JLo have…or even Mariah, for that matter. I don’t think the ability to sing really has that much to do with it. Look at Randy and Simon!
Although I don’t really know any of their personalities well enough to think they’d make an interesting judge, I do know Chacka Kahn and Aretha Franklin are both pretty out-spoken and frank so might do a good job but would both probably be considered too old and not in the demographic they are typically aiming for.
They ought to consider having a "guest judge" each week and rotate the ones you mentioned. I’ll bet THAT would bring in a lot of viewers when their favorite singer is on.
But, a lot of good ideas, Bono. Maybe AI ought to hire you to help them out…ya think?