Make it so when a defender titan activates their bubble they get 3 or 4 overshields and a slight speed boost. This could be great in pvp.
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The problem is that Defender is a support-oriented subclass. If you buff it to the point that a solo Defender is balanced with other subclasses, then a Defender with a coordinated fireteam will be near-unstoppable. They're potentially scary-strong now: imagine having six Guardians charge out of a bubble with Weapons of Light active: two-hit kills with a Scout. One-hit body snipes. Most pulse rifles generate two-burst kills. See the problem?