I started digging into the Korean website 2 days ago, so I will try to answer you guys :
1. Beleive it or not but Legend and Hero modes were only implemented during season 2 and toward the end with the angels. So 6 months after second season launch. From Google translate, they made those modes for beginners to get easier better gears. So I can't tell you how we will be able to go to new content if we've to clear Myth first or not.
2. Nightmare difficulty wasn't released at launch of season 2 and I still didn't saw it after browsing 6 months of updates.
3. Raise level cap will start from 55 to 60 then go higher and higher with new released content for season 2 (thre like 5 parts of it)
4. Ally equipements slot was not in the eason 2 news that I saw (I still didn't finish to browse through season 2). Instead a jewelry sysem and enchancement ally skills were implemented.
5. We will get a new raid with each new zones, so far I've discovered 3 of them.
6. There is chance that Global version will have sooner some of new stuff, specialy updates for the UI because we had a lot of them that Korean players only had during season 2.
I'm in the process to publish news about these infos on my website, if you want to see some pictures, please visit my website from time to time :
https://fanofdragonblaze.wordpress.com/
There're a lot of infos so I will realese them a bit each day.