I don't think Delta is ever going to add Belize. Delta is in more of a pull-back than an expansion mode. And Delta stock has tanked.
However, if given regulatory approval, Delta and Continental are soon going to be working together with code shares and such, and their frequent flyer programs will be set up so that you can earn and redeem Delta FF miles on CO, and vice versa. So maybe I can use my 400,000 DL miles to fly to Belize on CO.