Gentle reminder: This is the For Beginners forum, so replies need to address the original question and not tangential details or minutia.
5 hours ago, thesargento said:
So my question is: Where to start to turn an MMORPG into a single player game?
In other words, you're trying to make a regular single-person game.
MMO is a term for games with massive numbers of concurrent players, generally around C100K. That is, around 100,000 concurrent players. About that point there is an incredible shift in infrastructure. Below that point it is something that can be handled on a small number of commodity machines. Above that point you need global infrastructures with large support crews scattered around the globe.
I've worked on games with tens of thousands of of concurrent users, and they were readily handled with a few instances of simple servers. They're the typical online game with many thousand dollars spent annually on infrastructure. With an actual MMO you're looking at many million dollars spent on infrastructure.
Since you're looking for a single player game, you're looking at the traditional typical role playing game.
The games you mentioned are called Diablo-style ARPG since that was the first game that popularized the artistic and gameplay style. The "ARPG" is Action Role Playing Game, which differentiates it from other RPG styles like JRPG, WRGP, Text RPG, D&D-style RPG, CRPG, Roguelike RPG, and more.
5 hours ago, thesargento said:
-I’ve been able to mount premade private servers.
-I’ve modded quite a few games via tutorials and just experimenting.
Have some knowledge of HTML, PHP, XML, Javascript and LUA.
Mounting servers is irrelevant as a single player game doesn't need them. Unless you're planning on making the game on a web server, HTML, PHP, and probably JavaScript will be irrelevant. XML knowledge may help as a data storage. Lua might be useful if you create a scripting system, but that is usually unnecessary in a single developer project.
These days any of the major engines can handle a Diablo-style game easily, and both the Unity Store and Unreal Store have ready-made packages that contain the bulk of the work for you, saving thousands of hours of work for $50 or less. The Action part of the ARPG style is the biggest difficulty in the style.
If you're using a modern engine the bulk of the work will be art and animation assets. Everything needs art assets, and everything that animates needs animations, usually multiple animations. You'll need to model everything in your world and animate all the things, create all your particle systems and effects, create all your weapons and items. Since nearly everything has actions available you'll need an enormous number of action animations.
The scripting side (if you use an existing engine) is not particularly difficult. There is a strong software engineering task of engineering systems that can interoperate with each other. You can't just cobble together a few scripts and hope they work together, a big game requires careful engineering. The better ready-made packages will have done all of that for you, and adding scripts is mostly just composing existing scripts and occasionally extending a script into something else. Those few scripts hook up to the animation data, and it is effectively the animation data that controls the game.
Both Unity and Unreal also have HTML5 project settings, although there are some with some serious limitations for performance and memory, and concerns with the amount of data that must be transferred. If you're looking for a web game you can potentially use the engines and still launch as a web game.
Building an ARPG game from scratch is an enormous project. Even leveraging a major game engine and commercial packages you're still looking at a serious amount of work.
