(I was going have this be a reply in the future tutorials thread, but it is more of a "what-if" feature idea.)
1. Another: play with the viewport size a little when a demo is playing.
A number of games close in the top and bottom viewarea a little for cutscenes (a few Quake mods like OUM and some others whose names elude me have "cutscenes" -- usually when you push a button to show what door or whatnot just opened).
Would be a cool effect for those as well, although only Nehahra really uses demos for cutscenes as far as I know. This would require a modhint of some sort (an extension or having the server play with a cvar).
As far as demo playback goes, the small amount of extra space could optionally be used for demo playback controls and/or even a demo progress bar.
2. I personally like transition effects or sequences. Maybe not a fit for a true Quake mod, more something like a total conversion would be to have something happen in the middle of the use of a teleporter.