Slots can’t be increased. Even if it was possible, I wouldn’t like such a change. We don’t have CoV and all the new buffs, right now it makes sense to keep the buffs like that because players have to make choices instead of just getting all the buffs.
One change that we might see is to “join” Might and Shield, Bless the Body and Bless the Soul and maybe 1 or two other similar buffs. This will probably be related to rebirth. This way roughly 2 buff slots will be freed which is more than enough in C3.