These are a few things that I think will be done for the fight, specifically how a possible SMN Egi would be done.

1. It won't be something that you get whether you beat him on EM or HM, it will be a specific drop after the fight.
Whether it would be from the HM or EM version I can't say, but I don't see it being something you instantly unlock after you beat him. As a SMN if it drops, you can roll Need on what I think will be a sort of "Elemental" Spirit Pact item. Everyone else would be able to Greed on it, but if there's a SMN and they Need, they automatically get it.

This will make him a little harder to get a hold of and will ensure that not every player with a SMN Job will instantly have him as soon as the fight comes out, especially considering what Role the Egi would play and how it would be balanced. I don't believe we will have access to him like that on day one though, a month or two down the line though for sure.

2. The fight will be a little similar to Garuda's fights.
This is all conjecture on my part, but I see it as a fight that will have an "Eye of the Storm" setting. The team and players will be based inside of a whirlpool, and Leviathan will be mostly outside of the dry area to make hit and runs and occasional attacks from the water, sometimes leaving it and going into the dry area to attack directly. Maintaining enmity on the Tanks or otherwise during this section. Akin to the fly-by attacks that Isgebind would do in the Stone Sigil.
If information contrary to this has already been put forth, go ahead and ignore this section in it's entirety.

Any thoughts?