Just saw this team and I noticed I have one extremely similar, I can definitely attest to the effectiveness of Gengar / Kingdra / Breloom. The main thing I see is the lack of a Steel type, which might not be 100% essential for a team as offensive as this, but it is very hard to play around Life Orb Latias / Latios, especially when you're using Hidden Power [Fire] on Gengar. I'd had a lot of success using Scarf Jirachi alongside that offensive core, and if you were to replace anything, I'd advise removing Weavile in place of it, as it does seem like your most replaceable team member. Jirachi similarly checks Dragons, while also helping you against Latias / Latios, but you can also Trick Volcarona to mitigate the few setup attempts it might get. You may argue that you don't want a Scarfer on this team, but I'd argue that Jirachi covers up a lot of the weaknesses your team has, and the change is worth it.
If your team is fully based around a sweep with either Kingdra or Breloom, there's another change you could make. I think you could try out Will-O-Wisp on Gengar over Taunt, as you still can get around Volcarona through Destiny Bond + Stealth Rock. Blocking Chansey / Blissey becomes less important with Taunt if you go with the Jirachi suggestion, as you can simply Trick off your Choice Scarf and send in Breloom. In general, I've found Gengar baits a lot of switches into opposing Scarfers, most of which don't appreciate a Will-O-Wisp, making it a hell of a lot easier to setup with your setup sweepers.