Well we could use the system that was popular during the "prime" of the RP Forum, it was created for a OP RP by a member that, I think isn't active here on OMF 3.0 (but was in OMF 2.0 where he created the system). Pitou had a very good description of it too and I think she is still lurking around here right? I can only explain it roughly, so for my details you could ask pitou.
It's basically a Stat system, but instead of stats like endurance, agility, intelligence etc. is a stat here basically your fighting style, your special weapon or your devil fruit (or all of them, since you can get more stats as the RP progresses). Then for every Stat you get initial 5 "Techpoints", with which you can build your "Techtree" and the techniques become stronger the more Techpoints you put on them. The Techpoints have characteristics, that are [Health], [Focus] and [Skill]. Where Health is equal to tanking, durability, endurance and also raw physical strength. Focus is everything to do with the mind and the senses, like a sniping skill would have focus as a Techpoint and Skill is equal to agility, speed, acrobatics, dexterity and the like.
So a Techtree would be for example:
Gomu gomu no Mi - A paramecia fruit that makes the eater into a rubber man, which gives him resistance to blunt attacks and can stretch his body.
[Health] Gomu Gomu no Pistol
[Health x2] Gomu Gomu no Bazooka
[Health x3] Gomu Gomu no Rifle
[indent=1] [Health x3+Skill] Gomu Gomu no Gatling
[indent=1] [Health x3+Skill x2] Gomu Gomu no Jet Pistol
[Healthx3+Skill x2+Focus] Gomu Gomu no Red Hawk (<- combination attack of jet-pistol and vulcanization, you add the highest of health, skill and focus together (so here only health 3, but it gets also focus additional)
And the Techpoints also give perks for a certain amount, though I forgot what they exactly were... basically the more health techpoints you have, the stronger you are physically also outside of the techniques, so no idea what it was for focus or skill. Like I said Pitou should know more^^''
I can understand if this is to complicated for you, though I always liked this system (if we can get the thorough explanation back)
edit: no idea how that with the indent works here o.o'
Edited by Raiden, 25 January 2015 - 10:04 AM.