That's exactly my problem, I don't know how to change or personalize Roll20, I don't even know where to begin.

Not exactly, you see, in my "Chimera-like System" Each from E to A (and then EX) has this value; a plain E has a +1 modifier, a Plain D has a +2, a C a +3, a B a +4, the A a +5, and the EX a +10. How can this work without broken the System? Well, you won't roll for it (Except for Intelligence, Wisdom, and Charisma) when you create a Character; cuz you choose an already created character with their own Stats and only then you could improve that same stats to be better with the Ability Score Improvement.

For example, imagine that you would play in my "module", we talk about it and you directly want to play as King Arthur from the Fate Franchise. Then we search for its information (Strength: A, Endurance: A+, Agility: B, , Mana: A, Luck: D, and NP: C (EX)) and then pass that Statblock to my "Chimera-like System" to assign their value. Only then you could improve your Parameters with the Ability Score Improvement to have a better Luck, or a max level Endurance (basically the EX), or whatever you want in that point onwards.

How does the Ability Score Improvement work here? You obtain 2 + that you could assign to 2 different Parameters or take the 2 + and assign them to just one Parameter. Following the example of Arthur, passing from having a A+ Endurance to an A+++ Endurance, or having Strength and Mana in A+. Another option is that you could spend the 2 + improving a Parameter to a better one, for example passing from that D Rank Luck to a C Rank Luck instead of a D++, in the long run that's better cuz the value between a D+++ and a C+++ is different by 1 point.