Time for some good ole wall-posting!

Here we go:

- More vast, hunh? You could say the fantasy planet is roughly earth-like, but simply larger?
As for a different seasonal pattern, with me being very familiar with geophysics and meteorology, it'd be a lot easier to simply say the seasons are influenced  by magic and completely abandon any kind of scientific explanation. Cuts down on headaches.

That being said, perhaps the seasons change more rapidly where magic pools together or areas through which is courses more strongly. Or maybe the weather is simply more turbulent? Or the season is attuned to the nature of the magic present?

- Well... What if our "heroes" exist outside the reincarnation cycle and--tying in to an idea later--can freely pass between our world and the fantasy world? That would be easy and fits nicely.

Also, what if the whirlwind of blossoms could just in fact be a torrent of magical energies swirling around our heroes. The scene simply gives the illusion of cherry blossoms. Perhaps due to fortuitous circumstances, our heroes actually experienced it while being in a grove of sakura trees?

I really want to push further this idea of weather being linked symbolically to magical events, at the very least in the fantasy world. Perhaps storms in our world are where magic leaks in from the fantasy world, and their path follows something like ley lines in the fantasy world world? Perhaps there's some subtle overlapping.

- While I do enjoy a certain amount of complexity, overcoming the hurdles of adjusting to a new body, new languages, and new cultures can be very daunting to roleplay through, so I'd agree on some sort of way to magically acclimate to the surroundings. 
As an idea, it could just be part of the transition process for special humans like our heroes. 
Perhaps, there are rules to the transition, such that the person must assume a form native to that world, or at the very least compatible with it. Moreover, basic knowledge (like "common sense") and native languages automatically translate through. Then the only roleplay difficulty would be dealing with the cognitive dissonance of having memories and maybe a body you didn't have before.
Our heroes may have the latent ability to change their appearance and form when they transition. It could be limited, so changing the form of one's original body is impossible.

- Continuing from an earlier concept: So... what if by magical happenstance, our heroes are swept away to this other fantasy world. And what makes them special is their ability to transition between the two without dying first.
However, what if they aren't the only ones who can do it? Perhaps they are swept up into a much grander scheme of secret societies or rival organizations of people like them? These groups would be something like secret agents with opposing agendas, but both working towards some flavor of "THE GREATER GOOD".

As a separate idea, what if technology and magic are competing forces? As the advancement of one continues, the other wanes. So in our world, technology prevails, and in the fantasy world magic prevails. Extending the idea further. If for example technology would begin to advance in both worlds, a backlash or surge of magic would hit both worlds as each distinct reality tries to reconcile the two forces into equilibrium. In that sense, the balance is very precarious and difficult to maintain.
This could tie into a more generalized [url=https://en.wikipedia.org/wiki/Technological_singularity]singularity event[/url], where civilization expands so quickly that the powers of industry outpaces the imagination and control of humans (or other dominating species). What if this trend is occurring in both worlds? The technologically-driven singularity being approached in our world, and a magically-driven singularity event  on the horizon in the fantasy world?

You could divide up factions of the aforementioned secret agents according to what beliefs they hold? 
Would their be singularity seekers? People who wish to see the pinnacle of civilization in both worlds at the risk of destroying them both?
Perhaps their are passionate zealots vying for an absolute balance, so magic and technology exists evenly across both worlds?
Is their even a correct way to the maintain the balance? What sort of alternative options might their be?

- Visual aids? Madame, I am bonified visual thinker, so please, dump as many images as you like. ;D

- As for the cast, we could just tackle the story from multiple angles using different groups of characters? Each group would have their own agenda and feelings. That way different facets of the story could be viewed through the lenses of a diverse cast.
Then you could just focus on the plot track that seems the most fun to work with at the time. If we get stuck on one, we can hop to another and try to unclog the story (if necessary) from a different angle.

- Post length I don't expect to be much of a problem for me. I'm happy with the "Less is More" style. Compactifying content can be difficult at times, but its pretty fun for me. Anything to reduce headaches down the road, I'm fine with.

- Hmm... I like the narrative your pushing with trying to juggle two lives in two different worlds. However, I'm not concerned that such a story would be too daunting to write for, but rather it would be too daunting for our heroes to realistically handle. I suspect that after a series of adventures and hardships, they'd have to make a choice to prioritize their life in one over the other. They can't be in two places at once with the ideas we've come up with so far. If that's the kind of consequences you're seeking, I suppose it fits. Though, bitter resignation to one's fate isn't really my cup of tea.

On the other hand, you could portray this rather simply with homesickness in increasing severity over time, even to the point that it becomes psychologically traumatizing. 

To take it a step further, what if because of our heroes newfound status as "horizon walkers" (or something like that) puts them  and the people they love in danger? What if they cant return to their old lives normally without putting their loved ones in great peril? Is it the rival organizations that drives them into a corner, or maybe their own powers?

Using the weather ideas again, maybe our heroes can only transition under certain conditions. Not quite something so difficult as "the stars must align in X fashion in the night sky during Y season", but you get the idea. Some routine cosmic or magical weather phenomena has to occur for travel to be possible.