No matter what game you play, whenever you roll the highest possible result, its usually boosted by something to make it nearly impossible to miss. In DnD its an auto-hit(which I dont agree with) but its still something to signify its importance. Yes, it deals extra damage right now, but what crit doesnt do extra damage. I'm talking about what I think of when I see a natural 20 and compare it into a Naruto based setting.
I see a 20 as a chakra boost/physical boost/strain on the mind in order to protect someone or something they love/are passionate about. I think, it would be nice if natural 20's offered a +5 boost to the ending result of the roll. My DnD group does something similar, except we make it +10 (A bit too high for a server like this). A +5 would make it so you could RP that burst of chakra/physical adreneline/emotional boost in order to land a move a bit more sharply. So that if its genin vs genin, if someone rolls a 20, the only way to beat it is to also pretty much roll a 20. Now why just +5 instead of auto hitting? Well it doesnt make sense, no matter the boost, for a genin to strike a jounin. Orochimaru whooped up Naruto in the chuunin exams, but with a boost of emotional power Naruto was able to at least stop orochimaru's summon.
Anyway, these are my thoughts on the manner, any other opinions are welcome.
Subject: Suggestion about Natural 20's 
Mon Feb 18, 2013 11:12 pm