I would personally hope for Fairy type effectiveness to be something along the lines of:
Supereffective Against: Dragon, Fighting, Electric,
Resisted By: Fire, Fairy, Steel,
Weak to: Poison, Ghost, Psychic
Resists: Fighting, Fairy,
For the attacking line, dragon is already confirmed to be weak to Fairy, I think it would be nice to have another type alongside Psychic and Flying that resist Fighting and can also deal with them; fighting is one of the most common offensive types used in the game, so I think this would give Fairies a pretty good niche to fill. Electric could use another weakness because unlike Normal, it has an advantage against water and flying types, so I personally believe it could use another weakness.
Fire is already confirmed to resist Fairy, and I was unsure of it at first, but I'm warming up to the idea if Fairies become checks to commonly used dragons. Your fire type would be quite a great addition to your team because of this. Fairy resisting Fairy seems like it should be as so(There isn't much basis for this). I've heard from multiple sources(And I believe I have read a fairy tale) where Fairy magick was reflected by metal/steel, which is where Steel's resistance to Fairy comes from.
I've also heard/read from multiple sources that Fairies are susceptible to toxins/poisons, and I personally believe Poison could use this to give it something to work with(I also am pretty fond of the poison type.) I personally want Ghosts to get another niche under there belt, and this would give someone a reason to choose a ghost type move over a dark type one(Unless Dark is SE against Fairy.) Psychic types already have two weaknesses(which if I remember correctly are both commonly used), so I think an advantage against something other than Poison and fighting would give it a nice niche to fill.
I don't want Fairy to be SE against Ice since Ice is already weak to three very commonly used types; I don't want Fairy to be weak to ice due to ice already being a great type offensively. I wouldn't mind if Ice resisted Fairy. Overall, types that I do not want to see weak to Fairy: Normal, Grass, Rock, Ground, Steel, Flying, Bug, Ice, Psychic, Ghost, Poison. I don't think normal should have another weakness without it having the advantage against another type. The rest of the types I listed already(With the exception of Ghost and Poison) already have 3 or more weaknesses. Rock, Grass, and Ice surely don't need anything else crippling them.
I wouldn't mind seeing the following types weak to Fairy: Water, Electric, Dragon, Dark(even though it's confirmed not to be weak to Fairy I would not have had a problem with it), Fighting
Water is my favorite type, but I wouldn't have an issue with an added weakness. Electric is up there as one of my favorite types, and I would not see an issue with another weakness. I never liked having to catch a dragon or ice type pokemon for dragon types. My water types almost always have ice beam to deal with them. I'm not a fan of Psychic type pokemon, and I'm not a fan of the fact that so many fighting type pokemon can use stone edge/rock slide.
Overall, Fairy is currently in my top 5 types; I fell in love with the idea when I first saw it, and it made me like Sylveon even more. Xerneas being a Fairy type just made me want X so much more, and I had already fallen for Xerneas. Swirlex has stolen my heart with its adorable face, and I'm liking Draining Kiss, Fairy Wind and Moon Blast so far.
Normal, Fire, Water, Grass, Electric, Rock, Ground, Steel, Ice, Flying, Bug, Dragon, Psychic, Dark, Fighting, Ghost, Poison