Like Muffin. That, at least, is one possibility, and it certainly sounds widely-appreciated enough to appeal to Google. A concept video, designed by Vladislav Vitula, gives us a look into what the author thinks Android 6.0 Muffin should feel like, and even dreams up new features: Multi-Window and Quick Reply.
In case you're unfamiliar with what Multi-Window is, it's a feature available on some Samsung and LG devices, and it basically allows you to run two apps side-by-side and simultaneously. As you can imagine, it has the potential to help multitasking, and is a welcome addition to devices with large enough displays (i.e. phablets) to comfortably visualize two apps at once. As for Quick Reply, that would simply let you answer incoming notifications right from a notification that appears in the status bar. Overall, the designer's love for Google's Material Design is quite obvious, as his conception has a ton animation-rich transitions at its core, and we found that we don't really mind!
Check it out (advice: watch at x1.25 or x1.5 speed):