I say:
Jennifer Hudson - You can make jokes but she has a powerful voice so I know she will have no problem paying tribute.
Faith Evans - she's great at giving tributes like below
http://www.youtube.com/watch?v=EutDyhJxldE
She has amazing vocals and is influenced by Mariah
Kelly Price - She's underrated and has amazing vocals that would compliment Mariah.
Leona Lewis - She basically is similar to Mariah to me when she first came out.
Dondria - She may be new and not main stream but I think she's good enough to go.
Al the vocalists I listed above were influenced by Mariah.
I would also put Jessica Simpson, Christina Aguilera and Fergie but I forgot it's BET