How about a good Humaniti book? For me Traveller has never been about the 4 arms humans (T20); Hunchback aliens (T4) and some of the weird humans from GT (although, some races in there actually make sense like the Yilians).
Failing that I would want a Droyne book that brings back some of the mystery for this most mysterious races. However, I, equally see the need for the Zhodani to be covered. However, the potential to get the Zhodani wrong is huge. The same applies to the Solomani. So, I would be very, very cautious about committing my money.
T4 with its Alien Archive and those above mentioned T20/GT races really rubbed people the wrong way. We want interesting but ultimately realistic aliens not rubber suit aliens or rubber nob aliens. We want something Ian Banks would find great as a sourcebook.
I trust you will not disappoint me with Aslan & later...Vargr.