First of all congrats on the ST8 win. You obviously know what your doing so feel free to completely disregard my suggestion which is to replace Virizion with a standard bulk up Breloom. It puts a stop to Rotom-W which could otherwise destroy your team, as well as preventing rain stall from toxicing you and stalling you out to put you into KO range for revenge killers. Overall though great team and once again congrats.
Congratz again for winning the ST8, you did well. Now i'm not really convinced by this team. Seriously, it's an original team but you're weak to a lot of stuff. First, you're extremely to sand teams, rain teams and especially sun teams since there is Dugtrio 99% of the time. Without Heatran (well it can't do anything) Dragonite just destroys your team. A volturn destroys your team. Terrakion ruins the team, same for nastycelebi@epower + psychic / giga drain (the most common set atm). I won't talk about SubToxicGliscor, or even, SDGliscor since Virizion is SD. SubHydreigon is a major threat, Latios too but you already know it, you have no switch for Mamoswine (hey foretress is weak, and it can switch...). Tornadus and Rotom-W just smash you. Yes, Rotom-W runs WoW and your Virizion (your only switch) is a physical sweeper. Alakazam is fucking annoying too with SR up.
You could possibly use SpD Rotom-W over Jellicent, he should do an ok job checking pretty much everything Jellicent does at the moment, and Rest could be used for some extra insurance; also being able to check some other trouble Pokemon like Landorus (no spinblocking sucks though, but you don't have huge amounts of hazards to protect mostly). You'll be more Fighting weak though for obvious reasons, but Mew, Dragonite and Forry should be able to keep most bar HP Ice / Stone Edge Nape under control.
Also the SD set I meant is that old bulky one with Ice Fang and Toxic Orb
sd lo terrakion is the only thing you seem hard pressed on.. so the only solution is possibly lowering its switch in chances. maybe a will-0-wisp on heatran > protect can do the trick. another option you could do is psycho shock > psychic on mew so it lures in latias and maybe LO latios (not specs). you can pierce through latias' defenses in particular and possibly give you an easier time with it.
last option.. you could speed bump tran with 16 spe or something to outpace those bulk up breloom before they spore you. pretty cool team still.
SD LO is worn down very quickly by recoil + tspikes, and I can always bail myself out with Sturdy Forretress. Not sure how Psyshock Mew would lure Latios in, I understand CM Latias though, but the Conk weakness it would create is not worth it. Ice Beam Mew beats CM Latias if Latias switches in on NP, so I'd rather try that than Psyshock.
I also run some speed creep on Tran but I took it out for this RMT.
Since you're Lati@s weak, I'd suggest using Shadow Ball over Flamethrower on Mew. Heatran can already check Scizor and kill it, and so does Jellicent. Even though it won't check the Lati@s problem completely, it'll be easier dealing with them. I really like the fact that you're using Jellicent as a spinblocker. As you said, it's really underrated atm, so it's great you're using it. Shadow Ball might seem like a nice check to Lati@s, but it's not enough (so again, SB on Mew).
SD Landorus can potentially destroy this team. Fortunately it doesn't get many opportunities to set up, but if you get caught off guard it can get +2 and OHKO pretty much everything from there. Since it's immune to Toxic Spikes, you won't be able to wear it down. You'll need to fight to keep the enemy hazards away, so that Forretress can maintain full HP for Sturdy so it can hit it with Gyro Ball. Although Virizion outspeeds Landorus and resists Earthquake, with sandstorm up it still takes 85-101% from a +2 EQ, so there's a good chance of an OHKO with sandstorm damage factored in. Since you outspeed it, you can hit it with a Leaf Blade but it won't do enough to pose any serious harm to it. Without replacing anything, a solution would be to add HP [Ice] on Virizon and change it a Work Up set, although you'd lose a lot of overall power. However, a better solution may be to replace Jellicent with physically defensive Rotom-W, although that would exacerbate your already existent Terrakion weakness.
Forretress can take a +2 EQ even with Life Orb. I'll try out Bluewind's Specially Defensive Rotom-W suggestion for this as well because I dont think you need a lot of defense to survive +2 Stone Edge anyway.
Ugh I had a long rate written up but then my internet went psycho for half a day. Ermmm... basically the gist of it was that Rotom-W really should destroy you if played well, it completely cripples every pokemon on your team with the right move. I get that you can often work around it by inflicting it with Burn (maybe if you need to, luring a Burn then switching to Mew would be a decent way to play) but it can still cause a load of damage before it dies, and regardless if it comes in on Forretress you can't touch it at all.
Its not easy to fix this unfortunately. You could always go for Gastrodon over Jellicent but having no spinblocker when using Forry > Ferro never works out well. I think the best idea is to do as Bluewind mentioned and run Rotom-C instead of Virizion. Either that or run Breloom to get rid of the SubToxic Gliscor 6-0 weakness and still fill pretty much the same role.
Rotom-W isn't a weakness in practice, my pokemon have the bulk to take super effective hits from it and my opponent really needs to constantly predict correctly for Rotom-W to do much damage. Breloom over Virizion is interesting but I really need the speed that Virizion offers to outspeed everything from Heatran to Hydreigon, and Mach Punch Breloom is too weak.
@ French - if you guys have suggestions please post them.