There isn't much information on OS X 10.11, but there is a little. None of it, obviously is verified, but here's what we know:
1. You WILL hear much more about it (along with devs) at the Worldwide Developer's Conference, which is in June. Looks to be June 8-12?
Moscone Center dates pin WWDC 2015 between June 8 and 12 | MacNN
2. Internally, the next version of OS X mostly likely has a "wine" code name, as nearly every version has since Panther. 10.10, for example, was called "Syrah," but I have no idea what 10.11's code name is. I can tell you only what's already been used: Pinot, Merlot, Chardonnay, Chablis, Barolo, Zinfandel, Cabernet, and Syrah.
3. While the OS releases are broadly speaking yearly these days, Apple may opt to skip a year -- or produce a mainly bug-fix "new" version that doesn't offer many features (Snow Leopard was famously the first of these). Yosemite being a pretty major upgrade, next year's might well be focused on refining it as Mountain Lion was with Lion. Or not. I know "Big Sur" is on the list of possible names, but no idea when that might actually be picked, if ever. It will most definitely be another California place name that holds some special feature or meaning to Apple, not just a town like "San Diego."
So yeah, look for the next OS X to arrive roughly in the fall (they are not slaves to a strict timetable), with an announcement and details in June, and a California place name again.