I would recommend ferrothorn because it would be able to cover your rock, water, and electric weaknesses. Also ferrothorn will add quite a bit of bulk to your team. I do find that rotom-wash and ferrothorn make a decent defensive core considering they cover each other's weaknesses so imo, ferrothorn should be on your team.
Edit: sorry for my ignorance, I just realized that ferrothorn is not in game yet. Another alternative to ferrothorn would be flygon(was gonna was garchomp but it's not in game yet either...) because it covers rock and electric and it now gives you a dragon type and we all know that dragon types are generally strong. Another thing i've noticed is that you probably want another special attacker. I would recommend noivern, but that just adds on to that rock weakness. Also if possible change confuse ray to will-o-wisp because burn goes a long way considering it cuts attack.
I just realized the perfect member(more or less) for this team, kingdra because with focus energy and scope lens will make it a criting machine. Idk the crit machanic in reborn but i know for sure that this kingdra set gives 100% crit in the current games