Mythics are extremely expensive, because they come with vast amount of unique content and reactivity. We've done updates here and there across the lifespan of the game, with a major chunk recently added for the Devil and some more still planned for Gold Dragon, but that's about as much as we can do within the scope of free updates at this point. Locking updates for existing mythics behind a paywall wouldn't really be received well, and we don't even consider it.
A new mythic path would be a cool addition, but it doesn't fit into the scope of a single DLC. It's worth at least 2-3 DLCs in terms of time and budget, and a gamble of "will such a large and expensive chunk of content sell well enough?" sorts is not something that we're willing to experiment with, 3 years after release.