In the past, most of my teams were a hodgepodge of favorites I had fancied bringing along for the journey. But, game after game... I'd find that I had used some pokemon (like Eevee and Blaziken...) much too often, and there were other pokemon I wanted to try out.
So, recently I tried a run with all bug type pokemon as I hadn't used some of my trusty bug mons in ages, and later I'd have themed, but nonspecific type teams like all birds which included of course many flying types but also pokemon like Golduck and Blaziken (good to see you again, partner). My current run on white 2 entails slithery, tentacled pokemon like Cradily, Elektross, and Tentacruel. It was nice to use Tenta again since I hadn't raised one in forever, and I forgot how fun it was to have it on the team. Also, I had always appreciated Ele's design, but never used it and now I really like it!
I put designs and themes before strategy. It's more fun, I think, to mold the best strategy with what you have, even if you aren't the most competitively viable team around, hahah. There are still some holes in type coverage my team is working on... but heck, I'm enjoying myself! And the difficulties you face with mono type or themed teams makes the game even more rewarding, especially on Challenge Mode.