Seeing as Thundurus has received the much needed ban, I thought I would share my opinions on the other pokes suspected.
Terrakion doesn't deserve to be banned in my opinion. Terrakion gains priority in the form of Mach Punch and Accelerock, not to mention an improved Stone Edge in Diamond Storm. I believe that there are a lot of viable checks to terrak, both offensively and defensively. Landorus-T, Celesteela, Sylveon, Swampert / Mega Swampert, Garchomp, Toxapex, etc. all can reliably check it. This is also without mentioning that pretty much all swift swim users check it. I believe that terrak is more of a nuisance if used properly then a banworthy mon.
I feel like my thoughts on this Gren change every other battle. Greninja gains and uses Nasty Plot, and either Water Spout or Steam Eruption primarily. People underestimate what Steam Eruption does for this poke imo. It allows it to be more accurate than Hydro, while still having the side effects of scald. It's almost a no drawback move, and can burn it's checks / counters. On the other side, it has fairly common answers such as Mega-Altaria, Chansey, AV Magearna, Mega-Venu, Rotom-W, Tapu Fini, and others. I think Greninja is actually very good outside of rain, and giving it rain makes it next to broken. There's also no more fakespeed to revenge it, since it's banned. I'd say im 65 / 35 for ban / no ban, but it's too close to simply ban it without a suspect. Ash-Greninja is also banned in AAA, which has more answers for it than stabmons :P
I believe that Pidgeot should be suspected. Being able to sleep something essentially without any drawback is borderline broken, and having base 121 speed makes it that much better. Unlike other sleep users such as Breloom and Amoonguss, Mega-Pidgeot can hit grass types and safety goggle users with spore. Even if grass types couldn't go to sleep, it still scares them out with Hurricane. It also gains Boomburst which helps it hit electric types such as Rotom-W, and hits neutral pokes harder than Hurricane. It suffers from 4mss, seeing as it has Sing / Boomburst / Heat Wave / Hurricane / U-Turn / Roost as moves it would like to run. It can definitely be checked by things like Tyranitar, Aerodactyl, Celesteela, and others. The only issue is by putting them to sleep, it puts pressure on the opponent. It puts the Pidgeot user on the aggressive side for at least a turn, and can nearly make the opponent unable to predict anything since it has to burn sleep turns, and switch into something that can eat a hit of its coverage. It also can live most priority including Banded Accelerock from Terrak at full 100% of the time (does about 90ish max)
tl;dr speed tier = free sleep, free sleep + hits hard enough with coverage + initiative + negates prediction to an extent = suspect imo
With the ban of Thundurus-I, I believe that the metagame will change quite a bit. I believe pokes such as Celesteela, Keldeo, Tornadus-Therian, Mega-Venu, and other pokes will receive more usage. I think this is specifically true to Mega-Venu, seeing as how rain is among the biggest things in the tier right now. I also believe that the ban makes bulkier playstyles a LOT more viable. For example, Thundy almost single-handedly made STABmons a tier without stall. Obviously there were teams, but it wasn't very common nor frustrating to play against.
Also, I want to mention a set I have been using and love a ton. It's life orb physical Tornadus-T! The 8 hp evs are there so that it lives 3 seismic tosses from chansey at full, it only costs 4 attack evs, but it's such a nitpicky thing. Dragon Ascent + Superpower + Knock Off is amazing coverage, and hits a lot of the metagame. The main reason i like physical is that it now has full power Knock Offs and U-Turns, which is very helpful. Dragon Ascent also hits hard vs offensive teams lacking a good bird answer. If you opt for Taunt > U-Turn, it helps vs bulkier teams (mainly stall). Flyinium can be used to not take life orb damage, and to nuke stuff. All in all, I think this is a neat mon on voltturn offense with potential in the metagame.
Tornadus-Therian @ Life Orb
Ability: Regenerator
EVs: 8 HP / 248 Atk / 252 Spe
Jolly Nature
- Dragon Ascent
- Superpower
- Knock Off
- U-turn