I'd like a badge which will be awarded when you earn at least 200 reputation each day for 10 days. I'd call it the King of the hill badge. Can be earned multiple times.
[This question predated the Epic and Legendary badges.]
I'm generally against badges that are given for doing stuff that'll earn badges anyway and earning the rep cap each day fits into that category.
It especially shouldn't be gold as some users will likely get one every 10 days almost without fail. It's the sort of thing that shouldn't be earnt multiple times either (IMHO).
I would support, say, a silver badge the first time you hit the rep cap in a day, but that's about as far as it goes (for me).
I fear it would only encourage some people to just post a lot of answers, hoping to quickly earn more reputation that day. Even though the current badges do tend to encourage to post often, they do allow for the occasional break to breathe a while and to give your typing fingers a rest. This is not a race, but a community where user input is encouraged.