Adding more skill isn't a problem as long as the ergonomy follows.
I'll go for some extrem example to make my point
Let say you had 5 different combo.
One who goes with 1,2,3,4,5
One who goes with 1,2,6,7,8
One who goes with 1,2,9,10,12
One who goes with 1,6,9,12,13
and finally one with 1,6,9,12,1



I effectively putted 12 skills here. However, you can squeeze that into a much smaller amount of keybind. Let's assume skill can change as they go through the chain of combo like in GW2.
First combo could be squeezed into one single key. You would advance through 5 different animation, every one of them doing something different (be it more dmg, dot, positionning requirement etc...) but it would effectively require you to only press 1 5 times. We'll call this combo A. The A combo is done by pressing A 5times, AAAAA
the second combo has a different path on the third hit but then stay linear. We'll squeesh all 6-7-8 key into one new B slot.
The B combo would effectively be done by pressing AABBB
The third combo follows the same rule as B, so we can have the C combo being done with AACCC
The fourth combo start with A, then follow with 6 which was the first number for B, and is followed by 9 being the first skill for C
The fourth combo then has two new skill that we'll squeeze into the D chain. Effectively making the fourth combo, ABCDD
Finally the last combo goes with ABCDA

In the end, we only used 4 keybind, A,B,C,D to to 5 different combo with a grand total of 12 different animation and effects.
By applying this system you could easily transform most melee combo into only 3skills.
For instance on the ninja, it could easily be squished into 3 skills.
AAA - main dmg
ABB - DoT
ABC - refresh duration
ACC - apply weakness

Doing so wouldn't make the rotation easier, it would just remove unnecassary skill bloat. doing 1,2,3,1,2,3,1,2,3 isn't harder than doing 1,1,1,1,1,1,1,1,1,1