I think 1080p is a safe bet (they don't want to compete with their own product), and it'll have to be a new chipset that hasn't been in a Nexus (at least historically that's been the case), so we'd be looking at either the Tegra 4 or Samsung octo-core thing (but I hope not).
Why not a Snapdragon? They just came out with new ones, right? Sure, but it's the same basic architecture with the same pipeline and not a "completely new" architecture. It's like a revision of what they released late last year (faster clocks and stuff). The Tegra 4 is an all new architecture for nVidia, and the GPU is fast enough to want some attention from Google for optimization. The octo-core from Samsung is also brand new for them, will undoubtedly be in a ton of devices (all from Samsung of course, which is a double-edged sword), and would also warrant some attention. But it just doesn't seem like it'll be as fast as Tegra 4. Besides, Samsung already does a ton of optimizing within Touchwiz for its chips.