Mastery bonus or do you intend to give a damage increase like the mastery changes? Since pretty much all classes shifted to % buffs instead? In general I like it that you wanna make Guardian's Ward impactful or needed to use again!
I am afraid if it will be mastery (with current overcapping) people might still skip this skill. Maybe something to think about!
Red Guardian is lacking in damage stats right now. They have to be compensated by a large number of essences, which in turn affects the damage mitigation rates, so any bonus to crits and mastery ratings will be important.
It's usefullness fully depends on what debuffs are coming from Bastion of Light. Since that is still a bit unclear I cannot say whether its good or not. Attack duration and slow buffs are not as impactful I am afraid.
Is it possible to make more bleeds from red to spread this way through Bastion of Hope? That would be great though!
There is also a debuff on outgoing damage from War-chant and a debuff on incoming damage from the red talent Rupture. I find the latter especially interesting to use with the yellow line talents.