Must have for me is the return for Naval Character generation. The expanded creation process for naval characters added so much to the game for me (same for Army and Airforce in Mercenary). The Classic Traveller High Guard with its super detailed assignments and branch layouts really made great character backgrounds. Battle, strikes, siege, shore duty, and special assignments. Command and staff assignments. Attaches. Cross-training. So much depth for building backgrounds. I miss that more than anything else from the old days.
The 2008 High Guard streamlined version was good too and I could live with that. Some combination of both versions updated to 2E would be fabulous. The only thing that seemed a little off in both versions, but better handled in 2008, was the Gunnery branch. I always thought it could easily be folded into the Line/Crewman or Technical branch.
So, Naval Character Generation is at the top of my list. BTW, why was it removed from the most recent version?