The new Samsung 512 GB eUFS embedded memory is about eight times faster than a typical microSD card, providing up to 860 MB/s sequential read speeds and up to 255 MB/s writes.
https://www.notebookcheck.net/Samsung-begins-mass-production-of-first-512-GB-eUFS-storage-solution-for-mobile-devices.268044.0.html
Phones like xiaomi mi mix 2 already come with 256gb storage. Therefore I'd be surprised if we didn't get at least a few flagships with 512gb by the end of 2018
I'd put my money on xiaomi and maybe one plus