I think it is going to be like when dark and metal came out. at first there was only special energies. Then the basic dark and metal came out.
Also new base set means new mechanics... So what about a new type and new basic energies ?
So long as fairies bring something new to the game. You may not notice, but each single type with its own basic energy has its own underlying strategy. Colorless's gimmick is that it can use any energy, and Dragon's gimmick is that it has to use specific energy.
Grass heal themselves and poison others
Fire have big attacks at the cost of discarding energy, and also they burn
Water has the ability to move energy around and switch opponent's pokemon
Lightning can recycle energy and paralyze
Psychic has the most status condition attacks
Fighting has coin flip type attacks
Darkness makes opponents discard cards
Metal have damage prevention
Dragon has powerful attacks at the cost of 2 different energy (although I wish the attacks were more powerful)
Colorless can use any energy
This is according to the official Pokemon TCG rulebook that you can download from the site.
I don't want to see dragon with its own energy, because it would kill the dragon gimmick. The dragons should indeed have more powerful attack. Something that would cost :colorless::colorless::colorless: or :grass::colorless::colorless: should only cost :grass::fire: or any combination for the same effect.
Fairy should not be a new type unless it bring something totally new. Something that isn't listed as part of the other 10 types. I also wish that each type is more strict with its strategy, that is, only fighting pokemon have coin flip combo risky attacks, and only metal pokemon have damage prevention, or only grass pokemon can heal themselves. So far, what I see is the types and their corresponding strategies put in a blender, so that every type seems to have every strategy. I want the game to make people have a reason to play a certain type based on gameplay mechanics and strategy, rather than it's because the pokemon art looks cool.
I have also heard another theory from someone that Fairies should be Psychic, and Ghosts are moved to Darkness.
Rock can also become metal too, since rock is supposedly hard, and have damage prevention type of attacks, so I'd rather see a shuffle than an addition of a new type.
Update:
I just had a realization. If they did the fairy type my way, then they could also do the remaining types that never got represented in the TCG.
Take Ice for example. It's energy will power up any :water: attack, but will give bonuses to cards that say if ice energy was attached, it gets whatever benefit, or would give bonuses against certain type of pokemon than if you don't have it attached. Ice cards would have a lighter blue on it's name portion of the card, while the rest of the card remains that same water blue color.
From this idea, I changed my mind about fairies being a sub type of every type, but now would be a sub type of colorless, or maybe psychic, if ghost moves to darkness. There would be still 8 energy types, but within each type would be special energy that gives bugs, or ice types, or perhaps ground types a boost, but such energy would give :grass:, :water:, :fighting: energy respectively. You can use bug energy to power up a grass type pokemon, but it won't do you any good.