1. Plan story arcs
For each beat in the story, set something up, have the characters work towards it, conclude, and then move on to the next thing. This will prevent you from having a comic that just meanders until something important happens, and it'll keep the story nice and tight and easier to follow for catch-up readers.
2. Keep art notes on important events and locations
If you're already writing about something that you're not gonna draw until months or years later, make sure you don't forget any important info about how things should look. If necessary, leave art notes for stuff you're gonna draw next week; it's always better to make things easier for yourself.
3. You don't necessarily need a large cast
It worries me a bit when people assume fantasy genre = more characters than normal. It...really doesn't have to. And I think when you force it because you think it's 'necessary' you end up with characters that are kinda just there to take up space, and it shows in the writing.
For example, the fantasy series I'm working on now started out with 11 named characters. Just eleven, in the entire story: 5 protagonists, 2 helper protagonists, 1 side character, 2 minor antagonists, and 1 main antagonist.
The sequel added...1 new character (the next main antagonist). The prequel I'm working on also adds just 1 new character...my point is, you can definitely tell a big adventure story while keeping your focus on just a few people...and no matter how many people you have in your story, make sure each one is important, and you'll be fine.
4. Do something unique...please
For every writer who assumes fantasy means 'gigantic cast, 95% of which is unnecessary and forgettable' there's another who assumes it means "travel across the land with Plucky MC and Sexy Elf Girl and their friends! And of course, Sexy Elf Girl can fight too; we're all about gender equality here~ 
There will be lots of little towns to save from mindless monsters based on DnD classifications (because we all know Dungeons and Dragons makes the rules for ALL fantasy) and at the end there will be a Big Boss Fight where the power of friendship (and/or Magic McGuffin #29087) will save the day! Yay!"
It just gets...so tiring to see so much of the same stuff. Sometimes people will try to be 'different' by simply basing their fantasy on Asian culture instead of European (because apparently there are no other choices???) but the end result is the same...it's the same tropes with different names.
Alternatively, they'll change Plucky MC to Brooding MC and try to make things 'dark' and 'mature'...and often they end up with a story that's just not entertaining at all. Like...dark stories are also supposed to be interesting; you can't just go 'world is hard and bad but I save people' for pages and pages and expect readers to stay engaged. =/ You gotta have at least one other thing going for the story.
Anyway, here's some random ideas I had to shake things up a bit more in the genre. Feel free to take one if you like:
-Don't use a flat-arc MC: So many fantasy stories have them...characters who don't change themselves, but instead change the world around them (see 90% of shonen MCs for a start). There's nothing inherently wrong with it; it's just very overdone.
How about a character who has to become a better person; who is challenged to do more than just 'get stronger'? Most people would not come back from a months-long journey exactly the same as they were when they left; why not use that??
-Change the structure: Instead of 'going to the big bad place to defeat the big bad boss', how about a fantasy story where characters are running away from something? Or one where their home is being invaded, and they need to stand their ground and solve the problem?
Or do like Lord of the Rings (the actual story, not the trope version people have in their heads) and have the cast eventually split up to conclude their character arcs in tandem, but in different places?? So many options go wholly unexplored...it'd be funny if it weren't so sad.
-Leave the girls alone: I'll chalk this up to the lack of prominence for female fantasy writers, but basically: the opposite of 'boobs on a stick sex object' is not 'girl who CONSTANTLY has to prove she's not a princess to be saved' (and then often ends up being reduced to a love interest anyway, but that's a story for another day).
Like...just leave the girl characters alone. Let them simply exist, the way all the male characters get to. They're not weird, they're not special, they don't need to justify or explain themselves; they're literally just people of a different gender. Hopefully, you don't need to hear this, but I'll bet someone out there does...
-Don't do a racism subplot if you don't want to: Speaking as a racial minority myself, I really mean this. If you don't know anything about racism, featuring it in your story will usually do more harm than good. You WILL make mistakes, and they will probably be offensive ones, because it's a complex issue that requires a nuanced understanding. 2 hours of Wikipedia research can solve a lot of writing problems, but racism is not one of them.
I think people are tempted to do it because 'it's fantasy, which means lots of characters of different races, which means racism obviously' but...you can just...not?? ^^; Racism doesn't have to exist; it really doesn't. If you can conceive of a world with magic powers that have no explanation, can't you conceive of one where it doesn't matter if you're a different color or if you have horns or wings?? Like, come on...
