Would be glad to see Magmortar leave because that thing is the devil. It and Jynx can both go to RU just so I don't have to sit there and get repeatedly wrecked by them (ok seriously Jynx you can stop using that 100% accurate Lovely Kiss on Lickilicky and getting successive 3 turn sleeps ok thank you). Rhydon is really really cool and definitely underestimated offensively; whenever I used it in Round 0, I only used a fast 3 attacks + SR set or a Rock Polish set, and both will be really good imo. I don't understand why people use bulky variants when it has such a beautiful base 125 Attack and excellent coverage. :(
Linoone leaving is good. If that gets in an untimely Belly Drum and you don't have Tangela, good luck beating that shit. Ludicolo is interesting because we get a decent check to Gorebyss (I haven't calced anything yet but being 4x resistant to its main STAB and neutral to the accompanying ones, plus with access to STAB Giga Drain it should be pretty damn good) but at the same time I envision that it'll be really, really, really good with Rain Dance. Ludicolo is almost assuredly going to be very high in April stats if it does drop down, just because it's going to be incredibly useful both on defensive and offensive teams alike. Sawsbuck is similar but it's really fucking strong and fulfills Miltank's role as Sap Sipper...er. Even without Sun, it's fast and hits like a truck, with the same Speed and an extra Normal STAB over Leafeon. I don't imagine Leafeon will go higher at all if Sawsboss drops... Also because I forgot, fuck Miltank it can go to hell. Curse Miltank is really awful. :(
Crustle is by faaaaaaaaaaar my favorite potential drop. An actually good, offensive Spikes user with decent STAB moves and moderate bulk? Please and thank you! I imagine Crustle will also get a huge jump as soon as it arrives, as Spikes are amazing and Crustle actually threatens a lot of spinners with its STAB moves (sans Wartortle...?). It'll be pretty fucking awesome and I guarantee you that, at the very least, it'll be on nearly all of my teams.
As for changes in the metagame, expect even more Tangela, really. You're losing two prime Tangela switch-ins with Magmortar and Miltank, and Tangela handles Rhydon and Sawsbuck easily, as well as sort of threatening Crustle. I think that Rotom-S and other Scarf users might slowly start fading away to other, more interesting sets, as weather becomes more popular and Ludicolo becomes a solid check to Gorebyss. Offensive weather teams will wreck house for a little while until people start discovering good checks to them (Rain Dish, specially bulky Ludicolo with Giga Drain and perhaps a way to hit opposing Ludi would probably be one of the best checks to rain, though I'm not really sure about sun yet). Spikes will become easier to set with more users available, and Wartortle will probably rise in popularity as it's one of the few spinners that can easily handle Crustle. All just conjecture, but I love talking tier changes so yeah!
edit: fcuking amarillo ninja'ing me. also forgot about crustle's smash set, probably will catch several teams off guard once its role as probably the best spiker available is secured!
edit2: other pokemon very near to that dropping range are electivire, medicham, and bouffalant - if any of them drop, i'm going to be so giddy. CB MEDICHAM HERE I COME