The nice thing about the "favicon" is that if you have several tabs open, it is easier to identify a web site. (specially if you have a lot of tabs open, and only the favicon is showing).
I think there should be a favicon. something like this would do, although bigger P's and smaller M's maybe. Competition time?!
Not great I know, but considering you have have something than can be rendered at 16x16 pixels and still be identifiable, using the letters PMMP could just work, done by someone with better graphic skills than me and more time to spend on it. Let's see something better!
The real question is, does PMMP have a logo? Like the nether reactor for the old. Maybe like a beacon for the new? Found one with a clear background:
I got the perfect one! Spoiler: What is it? A piece of redstone! (What shows in the inv. Not it placed.)
Why are all proposed favicons blocks that don't have proper functionality in PocketMine? Wise that they set the icon of Minecraft a grass block.