I think having a portable device that can be used for different tanks would be the best solution. Like a pH meter, but you can test everything with it by just holding a probe in the tank. It would have to have a small screen, so I wouldn't necessarily have to go back to my computer every time I want to communicate with this thing.
Measurements should be pretty exact (e.g. pH in .1 increments) as most manual test are not nearly exact enough and too much can be interpreted into result colors.
Continuous live monitoring and recording of all parameters would be neat for lab experiments but for everyday use I find it highly obsessive.
Things I'd like to monitor with it:
3. Hardness (especially kH as it can change faster than gH)
4. Light output (IMO rather unnecessary)
5. Ammonia (with an auto alert)
6. Nitrites (with auto alert)
7. Nitrates (like all other ferts with integrated calculator that will recommend how much N you'd have to add to raise it to a certain level that one needs to be able to set themselves)
9. KNO3 ?
10. KH2PO4 ?
11. Total dissolved solids
12. All other ferts (Fe, etc)
Decent pH meters are fairly expensive, so paying $150-$200 if I its portable and I can use it in all my tanks would be OK. More than that and you could only sell it to people with SEVERAL tanks, who really need the time savings of testing all parameters by hand. Cost is very important and you could rather include less features but make it more affordable.
Still though, it needs to be pretty simple to use, easy to calibrate w/o much maintenance and added reoccurring costs. A whole set, if I have to look at prices for all the different components I will need, I'm not interested anymore. Nevertheless, we don't have grant money to spend on this