Enabling stacking of all buffs would likely have a fairly deleterious effect on gameplay, which is why it was disallowed in the first place. Making this too easy makes it too easy to damage to soar past the ability of players to defend. You see this concept in situations like 3v3 arena, where players already complain that Kisame lineups are already impossible to defend against. Allowing stacking of all buffs would compound this issue severely.
I will submit this suggestion for you, but it is unlikely that the devs will implement it, both for the reasons mentioned and because it is likely coded in such a way that it would be difficult to change.
In the meantime, I believe there are guides in the other sections of the forums that go over which buffs will stack with others if you'd like more information on this topic.
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