Since dungeons are likely to be a thing sometime this year, I believe adding subclasses to the current classes would ideal. These subclasses should have uses for more specialized forms of combat, like enhancing allies stats, healing allies, defending or just dealing dps. If dungeons were added with the current system, I feel as though it'll be a little bland.

We have succession and awakening already, these fill the same role in the game that subclasses would, no?

These dungeon dont have any Trinity aspect, BDO have i-frame no tank and heal need. 
these *dungeon* will be simply Bashing Room with a puzzle for reaching the next bashing room.

Stop thinking that BDO will become WoW, its completely impossible and its must never happen.
These dungeons are nothing more that a 20 min grinding that you can do once a month. The reward will surely be at the same level that all other optional content.

most classes in this game are self sufficient with their dp buff passives and self healing skills adding subclasses would just end up being like a second awakening if anything in my opinion it would be better to ask for certain types of gear to be more viable in certain dungeons like what they are doing with hadum servers and adding unique weapons that help with the grind.

As a way around this While in a dungeon mob threat could easily be tied to the DP of the player.  The  player in the party that has the highest DP would by Default attract the attention of mobs more than the rest of the players by the % difference in DP stats.  While in a dungeon any non potion self heal would instead also be applied to all players.  

There is no need for "sub classes" to be used only when in a dungeon.