doublade is overdue, blissey is way overdue, and snorlax is very deserving of a move. those 3 I would agree on and don't feel the need to talk about at all since they've already been talked about.
Megapert is the next most important to happen for me, since it has multiple roles, all of which are incredibly well done: it can serve as an offensive SR user with the offensive SR set which is great and has been great since the start of ORAS, it can serve as a sweeper on its own with RD (a set which is incredibly threatening and probably most of the reason it should move up) and it can serve as a defensive check to dangerous pokemon like Entei, Chandelure, Rotom-H and Mega Aerodactyl with its defensive set which fits very well on defensive teams. Great pokemon, one of the best in the tier, should move up.
Forretress I'm not sure I agree with, I may I have to give it some more thought or I may have to be strongly persuaded in either direction. While it gives arguably more utility than any other pokemon in the tier in terms of miscellaneous support, it's so incredibly easy to setup on if you're not a fast sweeper weak to Gyro Ball, and it only has one move to directly deal damage, unless you run Toxic or Counter. Its passivity is what drives it into a wall in terms of effectiveness, because despite the fact that it serves as a SR user, a spikes setter, a spinner, a bulky pivot, and a Slurpuff check all at once, the utility it provides can all be overshadowed by simply how passive it is. However, if I had to choose, I'd say it should stay A rank just because it provides overwhelming team support, whether it be a suicide hazard setter on offense or a bulky pivot and hazard setter and hazard remover on balance or stall.
Meloetta is the last nomination, and again I'm not deadset on moving it up or keeping it where it is. It's a very good pokemon in the tier which I haven't had the ability to use very often if at all, but its SubCM set is probably the one that warrants it a rise. SubCM Meloetta is a very good asset to frankly any team archetype because it does so much work against stall, and while the tier is filled with balance-breakers, most people do have issues fitting genuine stallbreakers on their teams these days. Meloetta has brilliant natural special bulk, and when that is buffed along with its naturally high special attack it can tear stall teams apart. 100 HP means that it can create 404 HP subs if the Meloetta user wishes to give up speed investment completely, which allows it to sub on stoss from Blissey and break it more easily. Other sets, like Specs and Meloetta-P, can do good damage to offensive teams because it hits unnaturally hard. Its cons are that it doesn't have great natural physical bulk and its speedtier is lacking, and it has a crappy defensive typing. Not to mention it serves as setup bait for certain bulky setup sweepers, most notably CurseLax, who can boost beside her. I honestly couldn't choose a side on this move, just because it's a really solid pokemon but comes with its fair share of cons.
So, in short, I support everything but Forretress and Meloetta.