Rotom-heat and Vikavolt are both strong so it all depends on the coverage type you need, be it may a bug type or a fire type. Though for an electric type, Magnezone is the best mon of all.
If you go with Rotom-Heat, Fairy or a Psychic type would be fitting for the last lot. Gardevoir, Sylveon, Ribombee, Clefable etc. are all good choices.
If it's with Vikavolt, go for a Fire type for the last member. Arcanine is one of the most versatile out there. Can also be a fighting type coverage which you lack, having access to the OP move Close Combat.