In the section on building Villains and Extras, it gives a ton of options unavailable to regular players. Is there a reason these options are not initially allowed?
Yes. You should never make a game of the complexity to a RPG without considering very carefully whether an option *needs* to exist in the core game - everything needs to justify its existence or GTFO. My reasons for this particular decision:
1) Simplicity. You give people many more options than we did for Powers and it gets to be way too hard to pick a matrix of powers. Combine that with the literally hundreds of others configurations when building a character (gear x Traits x Attributes x Standings x Powers) and you run the risk of creating option paralysis.
2) Game Balance. Already people are inherently (some might say maniacally
) drawn to Mistborn's Powers, to the point that one of the top 3 questions people have about the game is 'Can you play a regular human, and if so, are you an idiot for doing so?' Tipping the game/world even more toward Powers would basically mean the effective extinction of non-Powered characters in all but purely "fluff" games. That does not fit with the narrative of Mistborn, which has really important characters who are not Mistborn affecting the story profoundly and often.
3) Character Balance. I made the conscious decision that having good Powers meant sacrifices beyond allocating Strengths. While I didn't want tell people they simply can't play Mistborn, I make players with Powers work for cool combinations via Advancements; a starting character with lots of Powers is not particularly good at anything, until he or she commits most if not all of their Advancements to getting stunts and improving their ratings. Mistings are good with their one power - the rating of 5 and a starting Stunt is a BIG deal mathematically - and normal humans' "power" is flexibility, both from Traits and freedom to spend Advancements however they feel is best.
NPCs don't follow those limitations, because they don't get the opportunity to grow through Advancements, and most only live for 1 adventure anyway! Thus, they needed a lot more options to represent the spectrum of abilities seen in the books.
Would there be any balance issues if a player were to take "A Rating 4 spike and an Average Power" as a Strong power?
I agree with everyone else that it's your game, and you get to decide what's right for your group. I don't think this is particularly overpowered as a Strong power choice.