You actually have a lot of good ideas, guys. It amazes me how the fans always seem to be more clued on what's best, than the professional teams hired by the artists. 
I agree that Mariah could definitely use a change in fashion and maybe even try a new hair style or something. She also should get rid of her creative art director (or whatever it's called); again, even the fans produce better artwork and most are not even pros. Hire a competent manager, if she hasn't already. Just an all around fresh team would be a good move.
Mariah's image is her biggest issue at the moment, from what I see. If she and her people could take care of that for her next campaign that would be a good first step IMO. Present her as the legend she is who's capable of standing on her own among current artists.
I'm kind of hesitant to talk about this since it's a touchy issue, but (and assuming she doesn't) it'd be nice if she worked with a vocal-coach consistently, or on her own – train the voice. TEC tour and then the Christmas shows have left it sounding the most robust it's sounded in over a decade. It'd be a shame if she didn't keep that going. As pressuring as it is, she can't afford more hiccups like the Rockefeller one.
As for her sound, she tried some new things (while still staying true to her identity) with MIAM, so if she could build on that and go even further, she's got nothing to lose. Look out for producers she hasn't worked with before, even underground ones or people outside of the US.
I also like the idea of cross-promoting... doing album tours and concert mini-tours at the same time. Focus on selling your music, your art. This is something I looked forward to with MIAM, but her personal life got in the way and the effort to sell that album's music, which was Mariah's best in years let's be real, went astray. Understandable though.
Her Christmas NYC residency should go on this year, needless to say. It was a hit on all levels. Maybe even bring it to the UK this year... do a couple of dates in late November/early December over there.
This year is the 25th anniversary of her debut (alongside the 20th and 10th respective anniversaries of Daydream and TEOM). It'd be awesome if things ran in a professional manner.