Yes, I replaced mine with an icon relevant to my niche, but faded down like the default one is (so it still somewhat encourages people to replace it with something more colourful and personal). Also ditched male/female versions.
I think the default question mark looks pretty confusing, especially next to posts.
What is your particular forum about if you want help with ideas? If it's the one in your signature, then your logo symbol - same as your node icon - would work really well, just remove the colour so it's grey and fade it right down.