Step 1: improve our current systems so mistakes like this don't happen anymore
Step 2: stop the sale of semi-automatic and automatic weapons. Allow current owners to keep their semi-automatic and automatic guns through registration. Not sure how I think we should handle the sale Of those existing guns.
Step 3: anyone who wants to purchase a gun must either attend a gun safety course or prove that they attended a gun safety course. Guns are serious weapons and so many death from guns are from people being stupid. If you can't pass a gun safety class, you cannot own a gun until you can. I think like drivers licenses you should have to be recertified every so often.
Automatic weapons have all but been eliminated from private hands due to a rider on the McClure-Volkmer act of 1986. You CAN still get one, but it's very expensive ($20,000 each) & there is a much more stringent background check. And they've been highly regulated since 1934. There has only been one instance since that time of a legally owned automatic weapon being used in a crime in the US.
Banning semi-autos will be a tough sell. More than half of all rifles and ~90% of all handgun sales are semi-auto.
Back to the original point, you can't just remove someone's rights on a psychiatrist's recommendations. You won't just be fighting the NRA; you'll be fighting the ACLU. But, we have to do a better job of reporting. The VA Tech shooter did have his day in court, and he was declared incompetent. But, the state failed to pass that info on to the Federal Government. Both sides of the gun debate have been screaming about this for years & very little has been done about it. Obama called for better reporting, but I don't know if it's happened yet.
Most people would like to see face to face, private party sales of used fireams (the ONLY legal gun sales without a background check) brought into the background system. Unfortunately, enforcing that would require registration & that isn't going to happen.
My idea is a national firearms owners card - but with a twist. Give it to EVERYONE (whether you want a gun or not) when they get their driver's license. That gets everyone into the database & doesn't single out gun owners. And let's face it, if you've been declared incompetent, you probably shouldn't be driving either. If you've done anything to make yourself ineligible, your ID is confiscated and you would have to go to the DMV for a new ID that declares you ineligible. This lets private party sellers know they're not selling to an ineligible buyer - and make it a crime not to check.
In the meantime, we need to allow access to the NICS to private party sellers at no cost.