Just a thought, but when players unlock ancient curses, there is almost zero reason to use the standard prayer book anymore. I just feel like that is wasted content after that point. From a game design point of view, I think it would be great to have pros and cons to both books. Let me give an example:
Curse Deflect = Provides 50% damage reduction against X attacks and can deflect some of the damage back to the attacker.
Prayer Protect = Provides 60% damage reduction against X attacks.
Curse Soul Split = Heals a player by 10% of the damage dealt.
Prayer Soul United = Armor increases by 10% of damage received.
Now this concept doesn't have to be what is shown above, but I am just making a point. Above you can see there are pros and cons to both prayers and ancient curses. I feel like this could add more variety to builds that people could make. I have heard people say that, "Ancient curses are suppose to just be an upgrade," but if that's the case, why not upgrade the standard prayer book with higher level prayers on completion of the quest too.
Anyway that's it. I would love to hear what other people can come up with in relation to this topic.