Just a few things that are necessity from my experience - although I'm not sure any of them can really be invented. :)
A pacifier holder - to keep it in the baby's mouth. To be honest, in moments of frustration, I've thought about putting a rubber band around the kids head for .4 seconds, but I do realize that would not be safe at all. (don't worry, you don't need to call Child Protective Services) There is nothing more frustrating than having to pee sooooo bad, and having a baby that is almost asleep but starts to cry every time the pacifier falls out. You know if you go to the bathroom, you'll have a screaming baby and be back to square one (where you were an hour ago, with a tired baby who won't go to sleep).
A What the Heck is Wrong with Me? Monitor - it sits near the baby and picks up "vibes" so we parents know what the heck is wrong with the baby. Some options could be gas, reflux, boredom, wet/dirty diaper, I'm mad at you for something, tiredness, etc.
This should also have an "Yes I'm done pooping" alert as well. No one likes to be pooped on (just ask my mom, my brother pooped in her hair one time, don't ask my why her hair was anywhere near his naked butt) and no one likes to have to change a diaper three times in 10 minutes because you thought your kid was done pooping. It's not so bad with cloth because at least you know you're not just throwing away money, you can wash and reuse. If possible, an "I'm about to pee" alert would help, too, because I already took a shower today, thank you.
A car seat with a vibration feature - this one comes from Mike. All I can say is the vibration feature on any of our baby equipment is fabulous, it's basically a Quiet button. For whatever reason, Ezra looooves to be shaken up. We have been in restaurants and stores with the baby in the car seat and he starts to fuss and you know he won't remember what the pacifier is until he's screaming bloody murder at which point people start staring and talking. Taking him out of the seat makes it worse because you have to bend his arms and apparently he doesn't like that - all we'd need is a quiet button.
A handle for the bouncy seat - because no matter how many times I do it, I always pick it up by the toy bar which promptly comes detached so the bouncy seat falls on the floor - baby not included, obviously. Those things are nearly impossible to carry one handed, while carrying a baby (and possibly some laundry).
A spit-up measurer - every time we go to the doctor they ask me how much spit up he has, and all I can think is I have no freaking clue, I didn't have time to grab the measuring cups to catch his spit up in. If I was in a cranky mood, I'd probably say "enough to run about 6 inches down my shirt" or something to that effect. They usually throw out something like "a shot glass? a table spoon? a mouthful?" and I just pick one because, really, I have no freaking clue.