Hi There 
I evaluated some relays with newly assigned (red) guard flags. All of them had already the stable flag assigned. And (so far I could see) all of them had (almost) static IP addresses. In my case, this may be the reason why I don't get a guard flag. My ISP changes it every 24 hours. However, I'd be fine with "just" operating a fast middle node. 
I will keep an eye on this. 
Mike