There are many P&P RPG Games out there and if you want to reach the full RPG Community, it is important to be as connectable as possible.
At firstly you should check the most popular RPG systems as Pathfinder and D&D then you should also check the 5E system and savage worlds, these are especially good because these systems are connectable with many RPG titles.

I am on the way to work right now, if you need some more ideas please send me an email address I can use, so I can collect some ideas and send them in one email.

Kind regards

Amin