I though about it and each legion is slightly different, like warsmiths of the iron warriors, warp talons of the night lords, zealots of Lorgar, and the chosen of abbadon, what are yoir thoughts?

Warpsmiths are gonna go the same way of Techmarines, post launch.
Warp Talons are not a Night Lord exclusive unit.
Id assume you mean Dark Apostles when referring to Word Bearers, those were talked about by the devs and will probably be in.
The Chosen of Abbadon are actual leaders of monotheistic warbands within the Black Legion, they are way and beyond in power in comparison to anything that will rear its head in the game, so no i dont see them being a unit of any sorts, wouldnt even fit the heroes and the Chosen are named individuals and only number in 4 total at any time.

All that said, considering how much talk was on what the Apothecary equivalent will be for the SW, and how much Brent participated on that end saying they are wrapping their heads around it, there will surely be some sort of a legion specific class roster.

Yeah i don't know much about heretic legions, so i took a stab in the dark

None of the legions really should have specific classes, so much as specific abilities.
Iron Warriors should have the ability to entrench in better cover than others/or ignore enemies' cover.
Night Lords should have some sort of stealth ability, to allow them to set up ambushes.
Word Bearers should be able to inspire units around them(buffs of some sort).
Black Legion should be able to do any of those things(but only one at a time), but not as well as the more specialized legions, i.e. Iron warriors completely ignore cover, a black legionnaire who unlocks it and has that ability active ignores half cover.

Legion abilities will destroy any hope of even beginning to balance the factions before the game is even released.

We need to stop worrying about how fluffy the game should be made and worry more about how fun the game can be made.

Isnt fluffy fun?

Not if it creates imbalance in the factions.

Take Warpsmith's suggestion about having the Iron Warriors have a racial ability that gives them "Ignore Cover". Iron Warriors will be the go to faction for assaulting and taking critical points.

Basically taking whatever points they want whenever they want to take them.

How many times will you take a point then see a wave of Iron Warriors picking your forces off through cover and behind walls before you say "Screw this we have no chance against them"?

Its a two way street though, like BA could go rage and counterattack

All that would do would change every battle into a Blood Angel (defender) vs Iron Warrior (attacker).

But thats why there are 4 factions with 4 very different armies each