Not necessarily make a new class. You could add it into an existing class, such as Game_Actor. Then you'd need to modify the damage calculation so that it is taken into account with the atk.
The base_atk method in Game_Actor is the one you want to change most likely.
#--------------------------------------------------------------------------
# * Get Basic Attack
#--------------------------------------------------------------------------
def base_atk
n = actor.parameters[2, @level]
for item in equips.compact do n += item.atk end
return n
end
So something like this would do the trick, and I will leave it up to you how @atk_lvlplus is updated.
class Game_Actor
attr_accessor :atk_lvlplus
alias mawmkaesr_init_selfupgrading_weapons_8jk3 initialize
def initialize
mawmkaesr_init_selfupgrading_weapons_8jk3
@atk_lvlplus = 0
end
alias wpnmstr_kma_wpnugrd_bs_atk_84g2 base_atk
def base_atk
return wpnmstr_kma_wpnugrd_bs_atk_84g2 + @atk_lvlplus
end
end
You'd want to modify level up in such a way that this bonus is increased though.
I do have a question though - why not just increase the level growths of the level, rather than the weapon?