A way to define free-roaming world areas which can act as a sort of menu of things to do during a game loop, or as a clue-gathering mechanism.
Screenplay-like scriptable scenes (such as in Visual Novels,) which can reference separately defined characters, locations, and even camera angles.
A time system where areas have different properties depending on whether it is day or night (for example.)
An optional date system where the calendar progresses, which can be used to advance the plot.
A sophisticated inventory system, which understands stackable (quantity) items, containers (which can be opened, closed, persistent, or ephemeral, and have limits on what they can contain), expendables, key items attached to specific quest, items with internal tagging so that they can be used interchangeably to fulfill plot purposes (i.e., matches, cigarette lighter, and flint and steel all have the “fire” tag.)
Support for cross-platform deployment—in browser, mobile app, or as a desktop application.
Support for various rendering styles—text only (command line, browser-based, or wrapped into a user interface), or fully visual (either tile-based or backdrop and overlay image-based), rendered via Canvas, WebGL, or OpenGL.
Support for single-player games where you control one or more in-story characters, and potentially gain NPC companions—characters who adventure alongside you at times.
Support for socially interactive multiplayer environments, with chat, trade systems, friend lists, groups or adventuring parties.
Support for various scores, leveling systems, in-game currencies, and in-game stores with options to buy or sell items.