My thoughts at a glance:
What kind of team is this? I see 5 possible sweepers and one support lead/wall. Is this a Hyper Offense team? Tell me this, when you face your opponent, what is your teams objective? By that, I mean, what is your plan to win? Stall teams objective are to set up status or set up hazards around their opponents team and abuse efficent walls with type synergy in order to stall their opponent. I see your team, and I think to myself, what is their purpose. HO teams purposes are simple: outspeed and overpower their opponent. However, I see Whismicott and I see Magnezone which have no place on a HO team. I am going to have to assume your running a balanced team, since that is the average, and I see a lack of walls. Whismicott and Magnezone are your possible walls, and they share a weakness in fire. Weavile also hates fire, and that makes your team prone to sun (which I happen to see you run sunny day on Whismi).
I see; Magnezone, Haxorus, Volcarona, Weavile, and Starmie as your 5 possible sweepers. I see you mentioned Reuniclus as a threat, but, if you can set up with Volcarona before hand, it shouldn't be a problem at all. You also have too many sweeper for a balanced team. You need like 3 maximum, and so, I suggest taking out Weavile, Magnezone, and Haxorus. That should leave you with Volcarona and Starmie. They pack nice type synergy together. They form a fire/water core, and now, I suggest we put a grass sweeper to create a fire/grass/water sweeper core. I suggest Virizion as its Grass/Fighting type works wounders with Starmie, and, it also works wounders with Volcarona's typing. All three pack lots of speed, and can keep your offensive theme about. I see that Weavile really isn't doing much anyways, so you won't miss that. Virizion takes care of Ferrothorn, ect., Magnezone is useless now. Haxorus may have been nice, but you won't miss it because you all ready have enough offense on your team that he really isn't missed.
I see; Whismicott. Well, I already took out two Pokemon above so now we got some space for some walls! I suggest making a wall core that revolves around your lead, Whismicott (by the way, Starmie hates sun). Whismicott is a grass type, even though you have virizion, its fine. Now we should add some Pokemon to cover its weaknesses. First thing it first, Whismicott hates weather leads in the form of Politoed, Abamasnow, Tyranitar, and Ninetails. Politoed can run specs ice beam. Abamasnow packs Blizzard. Tyranitar mixed sets run both Ice Beam and Fire Blast. Ninetails packs STAB Fire Blast. Do not worry though, we can throw in Dugtrio to counter Tyranitar and Ninetails, your biggest concerns (as rain is helpful, and hail is weak anyways.). Dugtrio's Arena Trap prevents Ninetails and Tyranitar from switching. You can use this to EQ Ninetails (it outspeeds NInetails and T-Tar, run Dugtrio with sash.). Dugtrio takes on T-Tar too, as usually it tried to setup SR on the first turn. Lastly, we want something to deal with Dugtrio's enemies, and what better than Gastrodon? Ot has nice synergy with Whismicott in walling, and, it can fend off rain, and to an extent, hail.