Subclassing is great, it allows players do fill in the gaps in their builds by borrowing from other classes. The issue arises when there are no penalties in doing so. Take mythics as an example. They give you great power at the expense of drawbacks. It is up to the player to decide if they are willing to deal with the drawbacks. Subclassing two skill lines completely bypasses the pros and cons system. Players are able to make builds with high damage at no expense to their survivability. Using my templar main as an example: I gave up my Restoring Light for the sorcerer's Storm Calling skill line. By doing so, I no longer have access to my armor, recovery, purify, and healing skills and passives. This is fine. I chose to give up survivability and recovery for damage. With the current 2-line system, I am able to give up another skill line that I don't use, Aedric Spear for example, to equip a different line that has what im looking for in terms of survivability. I now deal more damage and am just as tanky, or even more, than before. This is not fine. There are no drawbacks to having 3 different skill lines. We need to limit to only 1 subclassed skill line. It helps maintain class identity and it lowers the power ceiling. There are combinations that are leagues ahead of others.
One other concern is the stacking of similar skills. I should not be able to stack the Templar's Rune Focus and the Warden's Betty Nerch for infinite sustain. Delayed skills such as the Warden's Shalks should not be allowed to be used with gap closers. This allows players to prepare to kill at a safe distance just to come in to kill in 1 second. That is to say that while Shalks is active, players should not be able to activate gap closers that deal damage or apply Crowd Control, or both, such as Ambush, Toppling Charge, or Dragon Leap. There is too much burst potential with little risk invloved.
Class identity has been shattered and we have yet to see any buffs to players that choose not to engage with the system. Perhaps damage reduction from players that use subclassing or even increased damage to those players would be a good place to start. Best of all, none of my proposed changes should affect PvE because they don't use gap closers or have the need to stack recovery skills.