If you're going to be hosting the game on something like
Roll20, you might want to pick a system like 5th edition D&D and stick with that rather than creating your own rules. Coming up with your own system, no matter how simplified, would be a lot of work that could be spent creating your campaign and helping new players get started. I used to run four-hour sessions once a week and I probably spent six to eight more hours a week getting it ready and working with players.
You can set things up on Roll20 so that, for instance, a player just needs to press a button to attack with their main weapon and the hit and damage rolls with all of their modifiers are handled automatically. And you can create maps so everyone can simply look at the screen and see where they are and what's happening. Also, as the DM you can make house rules that exclude complex game mechanics you and your players don't want to deal with.
This would make it a lot easier to start, and then over time your players would learn enough of the game that they would be able to join other campaigns or create their own; and they'd be able to use a wealth of resources like web sites, books, and spell cards. If you create your own system that's pretty much a dead end. I say this as someone who really did create an alternative to D&D many years ago and got paid a small amount by a game company who was thinking of publishing it.
