No problem. You’re welcome. I’ll try and answer some of your additional questions contained within the above post for you =)
To create assets you’ll certainly need a number of programs. Most artists don’t stick to one, as each has its own strength. You’ve already named several, including zBrush, Maya and 3ds Max. They’re some of the most widely used professional tools, but there’s also a number of free alternatives, such as Blender.
Traditionally speaking, most people - including myself - will use zBrush for sculpting uses alone. We’ll either create a model in Maya / Max and bring it into to zBrush, or create a mesh from directly within the application itself. There’s a number of ways to do that - the ZSphere method being one of them.
Quite honestly, I don’t know many people who actively use zBrush’s animation or rigging tools in a development scenario, especially within the game industry. I’ve never used them personally, so can’t comment on their flexibility, I’m afraid. I can’t imagine they’re as full-fledged as Maya, though, for example - which is widely used for this purpose.
My pipeline, and the way I work, is to create a low-poly model in Maya, which I can then take into zBrush and sculpt to add detail. After which, I take it back out and bake out the maps. This allows for the poly count to be kept low, whilst not skimping on the quality and detail of the finished product. There’s a large amount of people who use this workflow, and video explanations on the general process can be found all over YouTube and the like.
You’ll also want to consider texturing. This can be done in a number of applications, as well. You could use 3DCoat or Substance Painter which provide for an interactive workflow, allowing you to “paint” directly onto your model and even over seams with ease. There’s also the option of using Photoshop. The latter option also allows you to import your model and paint onto it directly - but, it is worth noting it isn’t quite as flexible or user-friendly as Substance Painter or 3DCoat, in my opinion. It can also be sluggish at times, to be honest. Photoshop is best for simply painting directly onto your maps, I find. And, it is widely used throughout the industry as a standard.
As I’ve already touched on, I wouldn’t animate or rig directly within zBrush if it were me. I can’t imagine the workflow is flexible, at all. You’d be far better doing this in Maya, Max or Blender. Though, I’m hardly an animator and someone else might be able to chime in on the pros and cons of this more.
SpeedTree is especially useful, I find. Though, it does have limitations. You’d be best using it for foliage - which, as the name suggests, is its forte. I’d not see much use for it outside of this scenario, though. And, unless you plan to work with its base trees alone, you’ll still want to bulk up on your modelling skills.
You, generally speaking, have the right idea. Don’t rush into it. Start small and build up your skills. Create a small section or part of the overarching world at first. Use this to test and even springboard the game into full development. There’s a number of ways to put it into action - through Kickstarter or similar crowd-funding platforms, for example. That, or even an indie grant.
It would be worth noting, that realistically, most people have a specific area that they are “designed for”, I would say. Some people make fantastic 3D artists, whilst some are better versed for programming. It’s unlikely that you’ll be able to grasp everything from the get-go, and some parts might not agree with you at all. There are, of course, exceptions and some people - many here on these forums - seem to be more than capable of balancing it all with superb results. But, don’t worry if there’s something you can’t wrap your head around - other people can and luckily, a fair share of them are available for hire! 
Outside of the artsy side of things, there’s the programming and foundation of the game itself. The framework. Unreal Engine uses C++, but it also has the user-friendly function of Blueprints, as well. This can be a fantastic starting point, and is a key attraction for many. Though, if you don’t plan to dive into any of this yourself, you could, again, hire someone to handle that aspect of the development. There’s even a MMO Starter Kit available for purchase here, which would no doubt be ideal to build upon.
However, how you wish to begin is entirely up to you. You can do it all from scratch or you can buy a number of assets to speed up the initial production, allowing you to get something workable in place quickly. It all depends on time and money available at your disposal.
As previously mentioned, if there’s something else you’d like to know, ask away and I’m sure I - or someone else - will be happy to help out =)
