1. Mountain Lion (obviously)
2. New iMac with spec bump, USB 3, TB, and ever so slight chance of retina display (but not holding my breath).
3. New 27" External display with new MagSafe 2 adapter, USB 3, and possibly retain display.
I really doubt that we're going to see iMacs or 27" displays with retina density yet. A slightly bumped TB Display with USB 3 seems very possible though.
If they did come out with a retina Thunderbolt display, I think it would be in addition to the current model since it would probably cost so much.
I definitely think we will see an iMac spec bump this summer. Just new CPUs, GPUs, and USB 3 like the non-retina Macbook Pro got.