Hello there,
as I understand it, using a 2* hero gives you 2 % of possibility to increase the special ability.
Using ten 2* heroes leads to possibility of 20 %.
I suggest that using more heroes increases the possibility more than simply adding it up.
For example:
1x 2* => 2 %
2x 2* => 4 %
3x 2* => 7 %
4x 2* => 9 %
5x 2* => 12 %
6x 2* => 14 %
7x 2* => 17 %
8x 2* => 19 %
9x 2* => 23 %
10x 2* => 25 %
Of course these numbers are just an idea and would also apply for all other heroes (1*, 2*, …)
Using heroes of the same color gives double the chance of increasing the special. For that reason, plus the extra 20% experience you also get for same color levelling, it’s usually best to use just the same color to level a hero over a variety of colors.
Using the same hero gives a 25% chance to increase the double, meaning using 4 of the same hero gives a guaranteed increase to the special.
It’s already very difficult to max out a 4* or 5* hero without maxing the special, so I don’t see any reason to increase the chances. Feeding 10 heroes at a time is already the best strategy.
Given the headaches in fully maxing a 3* special, I’d like to see an increase in the chances to raise a special to 8 once the hero is at maximum level in their final tier, but I don’t think it needs changing any sooner than that.
