Add grain to the adjustment brush, or add a slider sort of like the mask in sharpening that varies the grain by value. Film grain is not uniform but varies, as is it often looks fake.
Extrapolating this out to pie-in-the sky (Ideally combined with an earlier suggestion to let all the adjustment tools work together - gradient, brush, radial filter) - when selecting the mask tools allow all the adjustments to be used with the masking tools. The small panel that pops up would be eliminated and the look of the entire right panel would change (different color?) to indicate global adjustments vs being in a masked mode.
In the slideshow module bring back the toggle to require/not require a preview before the slideshow will run. It is frustrating to watch Lightroom crank away before it will start the slideshow, never had a problem with them being built on the fly before, and if there was a slight stutter between images occasionally it is better then the long wait before. I should be able to decide which is preferable.