First of all, there’s no magic answer on the supplemental content question. This is an area in which everyone is still innovating, big and small.
Supplemental content has three objectives:
- To drive people to the core content by exposing them to it through different channels
- To help make your universe “deeper” (I can’t think of a better word) than it can be just in on-screen episodes
- To create extra value for fans that they are willing to pay for
Any given piece of supplemental content can serve one, two, or all three objectives. But if it doesn’t directly serve #3, keep in mind that it is adding cost to your production. That may be OK. 1. is fine if it’s all about getting maximum viewers, money be damned. 2. is OK if it’s about the creative process or art for you. And of course 1 is cool if it ultimately drives monetization indirectly and 2 is cool if its upping the quality of content and thus getting more eyeballs and thus indirectly driving money.
Whatever the rationale behind the supplemental content though, it should be supplemental content that makes sense in relation to the core content. That means, at some level, it should always be about #2 if nothing else, even if you’re trying to make money! In your example, where you have a world that takes place in a video game, an online browser-based game is an inexpensive way to accomplish that. Of course, a mobile or desktop or console-based game is even cooler.