Fire Mage is a top tier spec at the moment, but I have no idea how the rotation works and it's based off of a burst window and then they do subpar dps for the rest of the fight.
For DPS it's been recommended to stack 3 of the same class as it makes it easier to handle cooldown management such as defensive abilities.

With that said. I think you should try what you envision your team to look like, what you think would be the most fun. Then tweak it from there. In this realm there hasn't been a ton of variables put forth and I'm hesitant to say my combo of Prot Pal/3xbm Hunter/Resto Shaman is the best when literally in solo ranking situation, Prot Paladin is ranked worst tank and Resto Shaman is the worst healer for Mythic+.
Prot War and Resto Druid are the current kings, I just haven't found a good way to managing their abilities.
The BM hunters also take quite a bit of pressure off of your healer with their spirit mend ability that you can throw on the tank.

So you bringing in a Lock to the mix gives the whole group a healthstone, that's a pretty beefy heal. Having the priest/mage gives the full 10% int/stamina bonus. You get a lot of value and it may be amazing. It's just an untapped area