I think corporate instances should be allowed only as hosts for accounts of their own employees. Letting large companies dominate the fediverse kind of diminishes the idea of putting control of social media back into hands of the people. If the companies really wanted to help the fediverse out they should be donating to fediverse projects rather than trying to monopolize it.
Hmm... I'm no expert, and probably not even competent at these sort of matters, but the thing that popped to my mind was "something something encryption something something trust". I wonder if this has a smart solution.