This is not to say some don't do this already, but this is my expectation for anyone who would want to be a witness.

1.- They need to be focused in two fronts. One would be oriented towards development, mass adoption and what have you, and a second one towards community and user retention.

2.- they need to stay engaged with their fellow steemians. via chat, regular posts, etc. If they seem like they don't exist, even if they are working behind the curtains 24/7 is not good for the morale of the platform.

3.- all top 20 witnesses i would vote for would have to be running a full node or a project of the same magnitude. They are not required, but i would expect them to operate this way, as they should understand the importance to do so, and could foot the bill at that point in time.