Drought Zard- aka "who needs that ninetales thing anymore anyways?" Sure, it's not a life orb, solar power beast of destruction, but it packs its own sun, shutting down other weather, boosting its own fire moves, and reduces the need for ninetails/set up for solar beam and solar power. The certainty of weather favoring it (reduce weakness to water, fire boost) along with 'significantly increased special attack' makes it a pretty formidable stand alone.
Mega Venusaur- Venusaur is pretty comparable to Blastoise as far as stats go, with a skew to higher spc atk. With beefed spc def and def, it becomes more tanky outright, thick fact halves the damage it would take from Ice and Fire, its main weaknesses, essentially neutralizing their type advantage. It's still a tough call to go with the bulk over chlorophyll, but it gives another option.
Mega Blastoise- where to start. Not sure I like the melding of both cannons into one big one. I definitely approve of the shelled guantlets/shields on each arm. I'd always envisioned something just like that for myself or a gijinka. I didn't like the added cannons to his arms on top of those shields at first, but then I figured they're kinda like Yellow's fist-shotguns and then I felt a lot better about them. Serebii doesn't say what stat changes it gets, but its Mega Launcher ability is kinda vague. It obviously would boost hydro pump and hydro cannon's power, but would it boost something like ice beam as a 'projectile' attack? I hope so. Blastoise has pretty poor atk and spc atk stats and could really use the help there, and doesn't get any justice from rain dish at all, and doesn't get anything to boost atk or spc, only boost defense. Not so much as a calm mind or bulk up is in his movepool and that's a shame.
Anyways definitely going dual water type with Froakie and Squirtle.