2Fruit has some brilliant ideas there :)
Poison should be super effective against water, cause poison pollutes water. Also, this would make Poison much better and popular. Just imagine water types being weak to poison. It would instantly make Poison like the best type ^^
Electric should be super effective against steel cause steel is a great electricity conductor. It's simple science. This way electric types should pair well with fairies and Skarmory would be a little bit easier to beat.
I don't agree with the other ideas cause they don't really make much sense to me.
Anyway, Dragons could be weak to steel, because knights kill them in tales. However, I wouldn't want this to happen ever, cause Dragons were already quite weakened by the fairies. And now they have 3 weaknesses which is sensible. Also, dragons wouldn't pair well with fairies then cause of their common weakness. So, bad idea.
Also, for the other guy who said Ice shouldn't be weak to Rock and Steel. That's because ice, rock and steel are all hard and inflexible materials. Now, since rock and especially steel are much denser and tougher than ice ,which is basically just solidified water, they can break it, thus the weakness. It's about what's harder. That's why Steel > rock > ice .