The aim is to take performance and scalability to the next level - while ensuring that stability and reliability increases. The SonicMQ guys have some great test environments which will enable us to provide even better quality code out of the door.
There's also some cool features that SonicMQ has like its Dynamic Routing Architecture and clustering that we are going to look to repurpose.
As ActiveMQ 6.0 is going to be a new architecture - it also enables us to look at supporting some more weird and wonderful wire protocols too (the invasive ones which force you to rewrite your broker to use them).
The development work is being done at Apache - with Hiram setting the pace at the moment. Please feel free to jump in and help - :)