That Gmail client looks sooo 1998. I don’t look to Google for any kind of design inspiration (apart from their icon design) but that might be just me.
For me, the Discourse design ethos stems from the logo
The shapes, the colours. Elegant. That shit is hot.
With that in mind… what about some variations on birdcourse and discat? 15 or so animal templates plus varying the cyan/yellow/red colouring and the outline colour would give a lot of combinations.
The fact they’re not squares makes them more recognisable. And, much like the Vanillicons you linked to, they also have a lot of personality.