I think this would be a bad idea since it would make things really complicated with the not very effective and super effective just for starters..

Triple resistance is already a thing. Look at Walrien.

And your acting like every pokemon will have triple resistance and on top of that i dont see how ghost type pokemon are broken especially gengar that has 3 immunitys. 

I know, it's just complicated stuff. 

It's kinda like Pixielate or Aerialate, which makes normal type moves into Fairy or Flying types respectively, giving an extra stab move with no risk.

What you want is simply add resistances to the mix

Edit: If they do something like your idea, I expect GF to expand those abilities rather than make new abilities all togeher.

Please no. The whole type table thing already got way too complex. I need the freaking table by my side to keep track as i play the game already. At this pace it will become a spreadsheet.

Nope

http://bulbapedia.bulbagarden.net/wiki/Same-type_attack_bonus

the italian page underlines how some Nintendo guides reported incorrectly that the bonus is halved by dual typing so I'll give you that...

Imagine all the problems to implement and balance that.

As I said in my first post this ability beeing broken or not solely depends on what type it gives to the Pokémon, say you want to give Ice to a Dragon/Flying type so that it only gets x2 damage from Ice, I don't see any Dragon Type using Ice moves besides Kyurem and Goodra so they wouldn't even use the STAB, or,  if you prefer, since the weakneses are not given to them they could just not give them the STAB aswell to balance things up, so the only thing the Pokémon gets are immunities/resistances...

Perhaps, seems they have to do something to stand out.

I want a 7th pokemon slot.......one for an HM slave. No more swapping one of my best 6 out to surf or cut a plant! Now that would be a revolution for pokemon lol

Imho the problem with HM is not that you have to teach them, but that they are STUPIDLY BAD moves besides surf and waterfall which you get much later in the game. 

Make Rock Climb a 90 BP Rock Type move, make Strenght 100 BP (wouldn't be used competitively because Return is 102, but it'd be enough for people to use it in the story), make Rock Smash a 90BP Fighting move and so on...

I would guess the original reasons to not have this is so that TMs still had some value while they were single use items but you could use HMs on all the Pokemon you wanted to, but now since TMs are multi use as well there is no real reason to keep the HM moves so utterly useless in power in places.