A common online environment where different video games intersect. People playing completely different video games can interact with each other there.
User, your favorite game character to interact with online friends that like different games.
Bring different kids together online.
The simplest implementation of this is to use an existing game environment such as Roblox, Minecraft, Decentraland, etc. and then just build from scratch the characters from other popular games for the purpose of making them available in the environment.
This wouldn't let people actually play those games, but it would provide the look/feel of a familiar game within the host environment.
For different games to be able to mingle they would have to be made following a set of universal standards that all the supporting video game makers adhere to.
When characters from different games enter the common area, they would lose some of the abilities/features that are incompatible with other games. This would protect the characters from some games from being overpowered by super-abilities of others.