I’m told Canon has been developing an EF-S fisheye for the last couple of years. It’ll be 8mm. We could possibly see the lens in the next few months.
Announcing a niche lens along with more mainstream stuff probably makes a lot of sense. What that mainstream stuff is, remains to be seen.
What about the 15mm?
From the same source, there is no plans to replace the EF 15mm f/2.8 Fisheye.