I too think they should have their own icon. As you said, @gaz_zippy , the type overlaps incredibly - so much so that it has got to a point that the Unknown type is literally that - unknown - aka all else  Perhaps Bonus caches should only be published as Multi's? Anyway, seeing the icon gives you little indication as to what the cache involves, as there are so many variations of the type!

But there really are a LOT of Challenge Caches springing up, and (after completing the challenge) it would obviously be nice to be rewarded with a shiny, unique icon  - this could also encourage more people to find these sorts of caches!

As you said, they have decided to focus their attention on Lab Caches, which seem pointless in every way possible...

Maybe we'll have a new icon one day? Perhaps they'll set up some sort of feedback site to listen to the wider community - oh, wait, they did that and they shut it down as there were too many suggestions they didn't agree with