That's more than a little broken, it is an interesting idea to make a feat for multiclassed characters, but with this it effectively means the character has taken something akin to epic levels in 3.5e, which isn't something really balanced for 5e. This is honestly stronger than Epic boons in the Dungeon Master's guide.

It doesnt add class features though. I do believe i need to rework a bit

So what exactly did you intend for it to do, if it isn't meant to add class features? 
Is it for abilities like say Warlock invocations that require X amount of level in the class to be an option?

Yes as well as rage uses and damage for a barbarian, sneak attack damage on a rogue for example. Its intended so you have to buy enough levels to get to X class feature before the level bonus for the other class applies to it otherwise there would be no reason to not multiclass into every class with this feat. In particular it was designed with a moon druid/barb so the PC could get access to higher CR wild shape while splashing barb for rage and at the same time aquire more rage uses and damage

It is interesting, not sure how many DMs would allow it, on the face of it, but it is very interesting.

I could se a DM ruling that taking this feat would only effect one of your classes, it they where to allow this as anything other than an epic boon, as it is honestly simply better than any other feat in the game.