The mods and options presented in Cortex Prime are probably more than sufficient to most system hackers busy. Occasionally though someone is going to get the urge to venture into the weeds to make foundational changes. Good examples of this are handling complications from hitches, and what you can do with opportunities. Even if these changes seem innocuous, they can have a big impact on game play.
When using standard difficulty rolls, a hitch normally means that a d6 complication can be introduced. If you tweak it so that the complication created is equal to the die that rolled the hitch, it makes hitches a lot more impactful. Imagine rolling a hitch on a d12. Now imagine you rolled that hitch, plus another. It would be an immediate takeout. If the intention was for the game to be really deadly and make players should think twice about engaging in fights, that might be desirable. But the first time it happens when it’s unexpected, the new rule may not seem like a good one.
With opportunities, players could be allowed to spend a plot point to “buy” the die that rolled an opportunity, removing it from the doom pool entirely. That likely doesn’t have as big of an impact as complications being created based on the die size of the hitch – but it does mean that on a string of bad rolls, players flush with plot points can run out the doom pool. That might be mitigated by instituting a minimum size to the doom pool, or other changes like enabling the GM to split or combine dice. The complexity added can start to add up fast.
These side effects and complexity increases are always pitfalls when hacking fundamental mechanics. As an example, I’ve used the “buy the die that rolled the opportunity from the doom pool” in play. It turns out that unless there’s a much stronger need to step down a complication or add an asset, players will go for removing the die nearly every time. They’ll also do things like only remove d8s or larger, leaving d6s. The temptation is there to add a minimum, or start introducing rules to allow the GM to step up or consolidate dice in the doom pool – which just adds to the complexity of managing the doom pool.
Of course, that doesn’t mean to avoid hacking the game to suit your needs! Just keep in mind that even a small change can have wide-reaching effects in the way Cortex feels and plays. Try to anticipate if those effects are desirable, and the mechanic actually meets the need you’re trying to address. And, of course, playtest, playtest, playtest. A mechanic that seems logical or good on paper may not work out quite so well once the dice hit the table.