He had been on an FBI terror watch list. If there were some rebuttable restriction that prevented such people from instantly buying assault weapons before an additional investigation is performed to clear them, then the Orlando attack — or a future attack — could have possibly been prevented.
You asked the question (sarcastically) what changes could be proposed, and I answered it. Of course the no fly list is not a perfect model, but it contains intelligence that should probably be shared with law enforcement/ incorporated into gun restrictions. It wouldn’t be crazy or illegal for our federal government to temporarily restrict people under investigation or labeled as at risk of committing a terrorist attack or mass shooting from purchasing assault weapons or large amounts of ammunition without undergoing an additional investigation. Would it prevent every shooting? Of course not, but the right to purchase or carry assault weapons isn’t even protected by the second amendment… so why can’t the government at least debate and possibly vote on such a proposal?