It would be nice to have a real buff system.

Each buff should be a toggle (instant, no CD, saved on loggout) and, when up, automatically applied to the whole group until canceled. EQ 2 did that, why can't we have the same here?

If you don't want some buffs to collide, use some kind of uniqueness per categories. If you want a buff to cost mana, translate this cost into a reduced mana regen. If you want to limit the total number of buffs, use a counter.

But having to recast them every 5 minutes is ridiculous. If you want a class whose gameplay is recasting buffs every 5 minutes, make one, but let that gameplay be specific to that class (Rift did that, there is a support class with a dozen of 5 minutes buffs).