I have a suggestion:
You could add those OP mods, but have a really expensive recipe to get access to it. For example morph you would need a powered morph core which you need to recharge before morphing back (Different mobs require different amounts of power, then they will change the power requirements as you use them). Another example is Mystcraft: To create a Mystcraft world you will need some kind of "Mystical power source" or something like that to keep the world up, or else if you are in the world, the screen will blackout and only the chat functions. You would need someone to power it up if you are in a server. If you are in singleplayer, you are basically screwed. If you are outside the world, the portal (or book) would not be functioning and if you still try to enter, it will still try to do it but it will play a mystical sound and if you are trying to enter by a portal, you get thrown out. If you try to use the bookstand, it will throw you back about 2-4 blocks.
Tell me what you think!