1) Lift the ban (however it is worded) on the CDC researching the problem and ways that might prevent these kinds of shooters.
2) Follow up on the FBI's thorough report on the School Shooter. Recommendations were made. Investigate which of those recommendations are being followed, which are not, why not, and where are the shooters still falling through the cracks.
3) Do an internal investigation of the local police and the FBI that did receive reports on Cruz and find out what were road blocks to intervention.
4) Obviously make getting assault weapons and large ammo clips harder to get. Surely some kid with the kind of issues this kid had would not have been able to get an AR-15 (or similar weapon) if it was a bit harder to do and he wouldn't have gone on this kind of rampage if it wasn't such low hanging fruit.
5) Like it or not gun lovers, there needs to be some kind of alert going out to the FBI with identified criteria depending on the findings of a decent study looking at red flags: depression and an accumulation of a small arsenal, for example, or accumulation of a small arsenal and bullet-proof vests. The red flag criteria need to be determined by evidence collected on mass shooters. The follow up when said red flags are reported need to be determined by research as to what interventions are likely to be effective.
6) Annually review these measures including a full review of any new mass shooters to update the plan.