[quote=@Pacifista]
I think coming up with moves on the fly is fine, depending on the character.

Luffy's techs pretty much amount to him hitting things, or him hitting things with a power up. Him pulling out something like Balloon or Axe or Storm for the first time isn't egregious because it's still within his known limits, while the Gears are a bigger deal because they change those limits.

I think the issues might come from characters suddenly being capable of things they haven't been established doing, or suddenly being capable of it. Usually One Piece handles it pretty well though. Usopp brings out new stuff every so often but pre-timeskip it's been dials with already established effects or gear made of store bought tools, while with Franky a lot of time was spent on Water 7 (and the Puffing Tom) establishing his capabilities, then again on Fishman Island. I forget the last time Usopp brought out a new Pop Green too for that matter.

I think characters like Scab and TJ are going to be less likely to pull out something weird compared to some of the more gear/tech oriented characters like Giko and Tenmon, but as long as we're aware of it I don't see that happening.
[/quote]

I pretty much agree with this. With some characters like Zoro, they're often shown in the background or during in-between segments on the ship doing things like lifting weights. I think there have been scenes of Usopp and Franky tinkering with stuff too, that justify some of their "I suddenly invented this new thing!" moments. And even though Luffy isn't shown training before introducing his first two gears, he does take notice of the Soru and Geppou techniques being used in an earlier confrontation, and his opponent I believe comments on how Gear Second is basically using Soru to create those bulges in his legs that act as the blood pump. 

I think introducing techniques that are simple punches, kicks, or otherwise "normal" moves on the fly is fine, but introducing something that makes specific use of a Devil Fruit power's properties--for instance, Zehst working his way up to iron-man style sustained flight--is something that they should be shown developing over time before using in battle.