But none of these teams have a theme. How can I Elite Four without prepping for type exploitation?

The theme is destruction!

good to see I had no overlap in my first conquest of the league with you folks. Shout-outs to Adam  for being the man courageous enough to use nerfed Gengar and a legitimately not-good water type. Shiinotic was a part of my rough draft for my team, but Tsareena chamed it's way into my heart because I rapidly realized that I wasn't about to have 4/6 members of my party be under base 40 speed and need to use Trick Room during main story to outspeed anything.

Here's mine in case you're interested.

Decidueye the Cunning

It had Spirit Shackle, Roost, Leaf Blade and Brave Bird but it mostly just fired off it's Z-Move and then got killed. Sad!

Persian The Triumphant

Persian was surprisingly useful to me throughout the playthrough. It's high speed and amazing coverage with moves like Power Gem, Faint Attack, and a Ground type Hidden Power kept it in the mix and it's generally really good at finishing off a weakened foe. The second pokemon I caught.

Hariyama The Immobile: There's a move called "Close Combat", you use it and everything dies. it's amazing. It also has Brick Break, Knock Off, and Rock Slide. It was the third pokemon I caught.

Garchomp the Unstoppable

Garchomp was one of two of my last minute additions picked up towards the end the third island during my playthrough. It's fairly quick and hits like a truck with moves like Dragon Claw, Crunch, Rockslide and Bulldoze

Crobat The Forgettable

Crobat was the second of my last minute additions and while it nabbed me a few key fights outside of the Elite Four it's contributions to the Elite Four were somewhat diminished. It basically either won me a battle single-handedly with moves like Air Slash, Leech Life, and Cross Poison, or it just got itself killed. The fifth Pokémon I caught.

And finally my most valuable member of my team.

Wishiwashi The Destroyer of Worlds.

http://img15.deviantart.net/0c67/i/2016/239/9/b/wishiwashi__daily__320__by_adrik_the_bat-dafj10l.png
Wishiwashi was easily my most powerful team member. It can nuke stuff with Scald, use Ice Beam or Bulldoze for coverage and... look, having Aqua Ring on it looked good on paper alright?

But Wishiwashi is probably one of the most overpowered pokemon I've ever ever used to be honest with you. I love it.

I had a team planned out based around Incineroar. I even started a game with Litten but some OCD addled part of my brain just couldn't accept not using the owl starter for a game called Moon (knowing I would eventually pick up Sun and/or Stars for a second run through). So I'm having to readjust on the fly to get better balance in my team. I'm only on the first island so I have a lot of work still to do.

I settled on a main team of Primarina, Lycanroc, Ribombee, and Lurantis early, and the fifth spot became Solgaleo's once I got it (though I didn't use it much out of fear of running everything over). The last spot on my team went to Arcanine when I fought the Elite Four.

I do wish that I explored including some of the Alola Form Pokemon more, but I was already pretty attached to my team by the time I got to the Alola Forms that I really liked. If I checked my Pokedex earlier and realized that I could get Pichu on Melemele Island, Alolan Raichu probably would have been part of my team. It gives me something to look forward to when I play the rumored Switch version or Moon, at least.

My final team was Decidueye, Palossand, Mimikyu, Drifblim, Marowak, and Sableye.  I had a Haunter and a Misdreavus, but I couldn't evolve them for lack of resources, so I had to cut them.  I decided on a Ghost team after learning Rowlet's final typing and seeing Mimikyu and Marowak.  Mimikyu was probably my team's MVP since it had X-Scissor to cover me against Dark-Types, and getting a free hit every battle didn't hurt either.  Palossand was great too once I got it Sludge Wave and Shore up.  Though, I didn't find much use for its ability.

Marowak was ok.  I trained the first one I caught, which ended up having Cursed Body, but I wish I had one with Lightning Rod, since I was mostly using it as a ground type any way.  I don't think it had one Ghost move on it at any point.  Sableye's Power Gem almost made up for its stats.  Wish I could've mega-evolved it sooner.  And Driftblim was ok.  Too many NPCs using Stealth Rock though.

Decidueye, Ribombee, Salazzle, Alolan Raichu, Bewear, and Wishiwashi were my final team. I had been training up a Alolan Grimer I liked, but it ultimately lost out to the Salazzle. I took a Lycanroc to Midnight form, but prefer dual-types, so he was cut too. I pick my teams with only single player in mind and basically reconcile aesthetic preferences with type coverage.

I think there are a lot of interesting designs in Gen 7. I really like the Ghost types in particular. I'll be getting Sun soon so I can run some different combinations through the man story gauntlet. I'm building my catch team with Lurantis and Shiinotic so they are already covered, but I also really dug Sandygast, Mimikyu, Dhelmise, Passimian, Toxapex, Araquanid, and Vikavolt, as well as both starters I didn't use.