I'm hardly an expert, but here are some of my thoughts:
If you haven't even started it yet, I don't know how concerned you need to be with sequels and spin-offs. It's great if you've got enough ideas that the potential is there, but any comic (let alone a fantasy epic), is going to be a ton of work just to get the main story done. I mean, can you ballpark how many pages or chapters you want your fantasy story to have by the end of it? And how many pages can you complete per day? You may very well have years to go before the first story is completed.
In a sense, every
comic is a "practice" comic, because you'll always get better than you were when you started it. You may very well complete your first chapter, then read over it afterward and wish you'd done things differently. I did that with mine. Heck, I did it like eight times or more. It may feel like a frustrating waste at times, but it will be good for the end product, because it will let you see how things flow, what ideas worked and what didn't, you'll get practice drawing the characters/settings, and overall just more practice, period.
I say just go ahead with what you're passionate about, rather than make something you don't really care for. If you are happy with it, sick with it. If you're not, re-work it until you are. Making a comic can require a lot of re-working and editing. No one ever gets it right the first time. You chip away at it, take things out and add things in, change characters' motivations and actions, develop your tone and your world, and in the end you just understand your own story better.