A somewhat wise person I know said to me a few years ago, "Rules are for players." What he meant is that the GM can ignore or modify any and all rules in a game as long as the result is more enjoyment for all involved. If a low level NPC wizard benefits from being able to cast Create Undead (possibly just before being eaten by a creation), Veil, a DM created spell or whatever then why not give it to them? If you need some sort of justification, then put the spell in a ring with few charges (just enough to make the PC who gets it some cool power and then bad consequences). But you don't need a justification -- the wizard may have gotten the spell from a spirit, demon, godling, dragon, elemental or something never seen before. In the end, the players may wonder how the NPCs (and monsters) are doing this or that and that provides a hook that can lead to anything the DM desires.
But I also think that the idea can be flipped on its head. Players that can be trusted (and most can) could be allowed to design new feats, domains and domain powers, spells and even class features (of course the DM has to sign off on all of them). With that sort of control, players can feel that their vision of their character is that much easier to attain. And the DM gets access to a treasure trove of ideas to apply in that and future campaigns. Win win. There are some downsides of course- the DM have to deal with power synergy (i.e. characters who become grossly overpowered) but that also leads to over specialization. Over specialization means weaknesses, vulnerabilities that the DM can use to show players why such synergy is counterproductive.
Applying this to Mutant Future, I would allow the players to come up with their own mutations and an additional racial trait or two for pure humans. We would also discuss technology and see what they would want to see in the artifacts that their players will encounter. (Hopefully those woulds will become wills soon.)
Rules are for players but that doesn't mean they can't modify them to suit the needs of the campaign as well.
