I've added a new design constraint. No overall modifier should be greater than +10. This keeps the math simple when adding numbers together. A die roll plus a value up to ten is easy to calculate by just about anyone. Using a bell curve the higher end values weigh a lot more so the +10 is more like a +15 or +20 in a flat distribution like d20. Since I use a 2d10 system with opposing rolls the lower end values weigh in a lot more and the later, higher values, add less with every plus. So 10 is pretty good in covering the full range of benefits.
More so, since most activities involve opposing rolls the bonuses can be added on either end and would result in a -10 to +10 overall modifier, depending on which roll the bonus is added. This bonus limit would apply to combat as well as skill checks.
Finally I also believe this cap sets a limit on the power a character may have. There is no "infinite" amount of bonuses a character can add to things like damage and skill checks, and every plus beyond 5 would cost considerably more and provide considerably less return. This will make player think twice about investing so much into something that will pay back so little.