To get some exposure for old ideas, how about having a scrolling marquee of random ideas (just the title or heading, not the whole thing). These one liners can scroll across the screen either on top, bottom or one side of the screen. Ideas can be clickable where the user can then read the details, comment and promote or demote. You can have this scrolling marquee on every page or just the main page.
This idea has been merged with "Random Ideas" tab at the top of the ArcGIS Ideas web page.
If not a scrolling marquee, how about a list of random ideas that constantly refreshes with new random ideas? Remember the old support site that refreshes with a new picture of a support analyst? Well have something like that but with a list of ideas that refreshes each time you visit the front page.
I am adding to this user's idea here: "Random Ideas" tab at the top of the ArcGIS Ideas web page