Poison
Fire/Poison
Ice/Rock
It would be interesting to see a good pure-poison attacker, which i see as crippled by the abundance of good steel pokemon. This would force us to get creative with the move pool, and lead away from a more offensive movepool and towards a more support/offense role.
Fire/Poison would be another interesting combo, as i feel it's a mixed bag on both offense and defense and would have certain pokemon that would be more automatically tailored to shutting it down (hi heatran and gastrodon) also still holding out for a fire ant pokemon.
Ice/Rock is an interesting typing that would provide awesome coverage, and a weakness to every priority attack (including 4x weaknesses vs the two more popular ones, Mach Punch and Bullet Punch) and would consider that over Ice/Steel, who's interesting niche is a bit more powerful in OU (dragon-killer) vs. a pokemon who's niche is being effective in both hail and sand.