Sure, I can help you create a simple text-based RPG game that can run on Android. Here's an example implementation using JavaScript enclosed in script tags within a single HTML file. However, please note that creating a vast open world RPG game like Ultima Online would require a significant amount of time and effort, and may go beyond the scope of a single HTML file. This example will provide a simplified foundation for you to build upon:
To elaborate on the implementation, this example sets up the basic structure of the game. The `startGame` function is the entry point of the game and currently displays a welcome message. You can add further game logic within this function.
The `displayOutput` function appends text to the `<div id="output">` element, simulating game output that the player would see onscreen.
The `processInput` function is responsible for handling player commands. Currently, it is empty and needs to be implemented according to your game's mechanics.
The code also binds an event listener to process player commands. Currently, it listens for input changes but can be modified to capture different types of input based on your game's needs.
Remember that this is just a simplified example to get you started. Developing a complete open-world RPG game would require much more code, assets, and design considerations.