I don't really think there is a need for a flag but if they really decide to create one for whatever reason, I really think that they should just stick with the current logo of Steem for easy familiarization.
Logo's carry power. It helps one to easily identify a brand or product. Creating a different one might yield confusion to others.
And so something like this: XD
Image Source: https://www.youtube.com/watch?v=YHtwNXcde3A
And what do you know, it looks cool! XD
I don't think Steem should have a flag.
However, if there a need for it, Steem logo on a white background should do