I have three dive computers. The best two, an Oceanic Pro Plus 2, and a Suunto Vytec, leave little to be desired while at depth, but there are a few items that would be pretty simple to do that I would like (I also happen to be a microprocessor controlled electronics design engineer.)
1. Logging. The Vytec and PP2 both have USB interfaces, and that's great, but it would be nice if they could remember more data. Most people don't log immediately before and after a dive, but rather much later after the trip. It would be nice if one could enter more log page info into the computer and have it remember it. Particularly air temp, dive type, and weight used.
It would be particularly cool if you could download a list of equipment from a PC via USB so you could check off which equipment you dove with and enter the lead weight you used and if it was OK. Weighting for different dive setups is one thing for which a log is great. Being able to download pick lists from a computer would be unique among dive computers as far as I know. As memory is so cheap now, and the USB interface is already a fact of life on higher end computers, it wouldn't take much more to make a lot more use of the feature. Such a feature could also be used to download a quick reference list displayable just before diving (last minute equip. check.), and while diving (like a wrist slate.)
Being able to record GPS coordinates before a dive, and compass directional info before/after & during would be fantastic. Don't need a GPS or compass built-in, just ability to enter and have it logged with the dive data. When you find something underwater, it's nice to remember how to get back to it.
The above would likely require a larger display like on the Pro Plus 2. It would be really cool if one could record a 10-30 second voice entry with each dive to enter reminders about whatever. That would download to the computer in an audio file identified with the date and dive number so it could be played back on the PC. Microphones and flash memory are cheap and durable.
2. A simple thing that would help me would be an adjustable minimum dive interval. When shallow diving with repeated surfacing for equipment change / stowage, the computers keep getting confused and logging multiple dives for what is really one extended foray. It would all be fixed if I could set the computer to "enter dive mode and stay there".
3. Another simple thing is to be able to review the last dive immediately on surfacing -- the Oceanic computers "lock out" the user for the pre-programmed 10 minute surface interval the computers use to decide a dive is "over".
How about being able to play Space Invaders while waiting at deco stops?