People keep talking about Mega Pokemon changing their types, but out of the, what, 12 Mega Pokemon we have so far, only one actually does so. I don't think that it's going to be as common as a lot of people seem to be hoping it'll be.
grass/fairy works for meganium. it give that line some much needed love.
fire/poison, or fire/ground for typhlosion.
water/dark could work (although seems redundant to have the only other crocagator line end up /dark) water/dragon maybe. Honestly the Ferligatr line is one of those lines that being mono type just made sense.
.....said an almost 30 year old married guy driving a van part time for a living 3ds my fc, 0602-6823-9646, my wifes 0146.9112.0071