I think the question of “what harm does it do“ is answered by “where do we go from here”? In the case of action moviemaking, it leads to the problem of topping what went before. You look at the Star Wars prequels, the Hobbit trilogy, and LotR and you see the damage it causes. Once Legolas jumped on the cave troll, then shield-surfed down the steps, the third movie is saying “well, we wowed the crowd twice, but we’ve done those things now” so they try to top the prior action with more spectacle (like the silly oliphant thing). Doing anything similar would just be ho hum. And this can eventually lead to the Hobbit trilogy of not enough story spread over too much movie runtime… and then fill in with action spectacle.
RPGs may not be quite the same, but Rule of Cool does lend itself to a sense of forced escalation because, while it may have been cool to do that thing once, it doesn’t really support that becoming a standard thing. Because if it’s a standard thing, it’s not doable under the Rule of Cool. This is, however, one area where narrative-oriented features may shine. With those, you’re not allowing exceptions to normal abilities because they‘re ”cool” and fresh. Those exceptions are built into the normal rules and are flexible and open enough to do what you need them to do. For example, Mutants and Masterminds allows PCs to use unpurchased power effects as “extra effort”. The player pushes their hero’s power in unplanned ways as part of the game system. It’s incredibly flexible but not allowable simply because it seems cool at the time. It’s an inherent feature and, yes, it allows you to do pretty cool things without leading to Top THAT escalation.
This is a fair concern: how do you address the possibility of "scope creep" or the like?
Part of my answer is to have several different overlapping or interconnecting focus points, so that the players are comfortable with the scope sometimes scaling down from one scene to another. Another part is to just be mindful and leave myself room to maneuver. Aim high, but not the highest I can possibly shoot for. A third part is introducing changes that arise as a result of the change of scope, which cause new concerns to develop that were not relevant before.
An example of that third seems warranted. In my DW game,
Undertake a Perilous Journey rolls became somewhat routine after a while, in part because the Ranger (who is currently on hiatus) was super good at them, and in part because we had three high-Wis characters in the party. I reflected on this for a long time, not doing anything specific (because I wanted the players to enjoy a period of doing well and riding high, as it were.) Eventually, I came upon the idea of adding a
fourth role to the standard three--a role that should naturally not be much of a concern for a
low-level party, but naturally something that a
high-level party, or at least
this high-level party, would start to worry about.
I settled on "Stealth" (which sadly doesn't have a nice occupational name like Trailblazer, Scout, or Quartermaster.) For low-level parties, it...pretty well makes sense that you don't really care about covering your tracks. You just want to get to the end of the journey as safely as possible. But for our party, at high level, where they have been drawn into much political intrigue and have to be careful about all sorts of information, getting to where they wish to go
without being followed or traced is in fact actually quite important now. It's not so important that it can't be ignored every now and then, but it's important enough that the party is cautious about it. The players were quite happy with this proposal, and the new presence of this role for
Undertake a Perilous Journey has added an extra layer of richness and complication to things.
Also, perhaps humorously, I have developed some house rules which do something not too far off from your "unpurchased power as 'extra effort'" example from M&M. For a "max level" (11th) character, spending XP can let you temporarily gain moves you don't have yet, or even gain moves outright if you have enough XP to spend. It's worked pretty well for keeping open advancement even past when DW would normally "end."
