Tips for an odd mid?

So I watched a diamond streamer recently who mains Sion mid. It seems pretty fun and I was thinking back to playing Mundo mid during placements last season. It was really fun and was at least doable bc he has high damage for a tank and could build resists and out trade and start diving at 6. Plus he can roam not easy from mid and tower dive with bot super easy.

Anyone have experience with it and can you give any advice? Max q first obviously, but then do you go w or e second? Also runes and masteries? 21/9/0 for mid damage and more kill pressure roaming, or 6/21/3 taking cdr and damage boost, defense bc you build it anyway, and ms for roaming?