I assume Mythic loot is handled as heroic is now. There were three theories I came up with based on tip toeing around the specifics of hardcore raiding at Blizzcon. Number 1 was pretty much denied on day 2 however.
1. Loot is handled similarly to Flex now. Each character has a separate opportunity to get loot on each boss per week. I wouldn't matter how many runs you did. Meaning guilds with 5 raids are on a more even playing field in terms of ilvl.
2. Loot is done similarly to how it was done in Dragon Soul LFR. Meaning the amount of drops are determined by the raid size. However, once a character kills a boss they may not again receive loot from that boss. This would allow for 5 mains 20 alts on first run to be brought. 5 new mains, 5 old mains, 15 alts on second. 5 new mains, 10 old mains, 10 alts on third and so on. This would still require 5-7 runs on week 1 but it would allow for each player only maintaining 1 alt. I feel like once a character receives a CHANCE on a boss per week it may flag them and not add to the loot total however leading to my number 3.
3. Loot is essentially the same exact thing as it is now. With the exception that bringing someone who has already killed a boss will lessen the chance of you getting loot on a boss. Basically it would let people "Help" friends raids but take away from their total loot. You would still need 5 runs of different alts to be competitive in ilvl for the first 4 weeks.
One other fact that makes me pretty sour is the fact that Heroic and Mythic are different lockouts. This will surely make killing multiple Heroic bosses each week a requirement for pushing tighter dps checks faster. Guilds willing to put in even more time will be able to clear content even faster than this tier by killing Heroic on week 1 and 2. Guilds who do not have the extra raid time will have a hard choice on doing progression or clearing crap content and the only way to judge whether or not to do it is attempting to math our dps thresholds on bosses 7 day guilds killed based on their ilvls at the time. This tier it wasn't so bad because the content was clearly tuned for ilvl 566 and no guild really had to worry about dps checks nearly as much as past tiers due to upgrades. Maybe I missed something that was said about it but I think this is the case?