Fantasy grounds New ruleset 5e compatible

[Master Wolf]

It is possible to make a new ruleset from scratch and play the 5e adventures? (I have bought all the 5e books from steam, is not to add content I don't have, but to add custom rules) Like adding my own 6 abilities, item properties, etc

[Zacchaeus]

Short answer is yes, depending on your skills in the needed programming languages and whether you have the hundreds of hours in time available. Usually though you don’t need to rewrite the ruleset but rather create an extension to change the existing ruleset to add what you want it to do. That would be a lot less work since you would presumably want to keep a lot of the existing mechanics.