This is something I've been thinking a lot recently actually
I've had a few ideas. The very first idea which was years ago was just a simple one. Colress with a Kecleon. My favourite trainer + pokemon. Sounds simple. Some of my more recent ideas have been a trainer sprite of Paul McCartney from The Beatles with a Dragonite. (Reasoning for this is so a certain friend of mine can steal it, we're both big fans of the beatles haha). I've also been thinking of an extension of the Help Staff custom avatar of sorts as of recent, but I don't have a strong idea of what I'd like it to be.
Another idea was a final fantasy X pokemon avatar. I'm a big fan of FF14 and some of their sprites they have for a side thing of FF. Random ideas come into my head every now and again, some of which I could very well make myself - I just can't be bothered LOL