One thing I would like to see from witnesses is some kind of star rating / best practice / mark of approval for the safety of alternative platforms and applications. I did read one post last year that suggested the process used to access one platform sent the keys to a server, and that this was both unnecessary and a risk (if not necessarily malicious).
I know that there is steemconnect, which a lot of the platforms use, but this does not always seem to be the case.
This concept falls into the general category of "doing the technical checks to protect non technical users" and, whilst this isn't the witnesses job in itself, it requires (a) trustworthiness and (b) strong technical skills. These are basically the two main characteristics that I look for in a witness (along with your no. 4 - Providing tools and services that improve the platform).
Potentially the concept could be expanded to outside exchanges and other crypto related affairs, but the main concern is keeping our own house in order. One of the worst possible outcomes for any blockchain is some kind of hack or scam or weakness that results in significant financial loss.
Yea definitely and I think that kind of falls under my #5 point of preventing / curbing abuse and scams - even though this not have had malicious intent it could potentially be used maliciously in the future so it needs to be stopped to help prevent bad things from happening.