I have been working on this now and again. Under the current version of Traveller, you can find several books which I have found invaluable. In my case it is converting the old D20 SRD over to Traveller. All of the items I reference are licensed for Mongoose Traveller
On races/aliens
Traveller Core Rules has the basis for notations for races (Strong/Weak Characteristic, Feral, etc.)
Samarden Press has Flynn's Guide to Alien Creation has a bunch more.
Supplement 11 - Animal Encounters has more for the workings of "animals" or non-sentients.
On equipment
Supplement 4 - Central Supply Catalog has plenty of low tech armor, melee and ranged weapons
Book 6 - Scoundrel has rules on tech familiarity should you want to introduce "high tech" items
On "magic"
Mongoose has additional ways or enhancements to having "magic" or powers
Book 4 - Psion and Judge Dredd deal with more psionics
Strontium Dog has an entirely separate system for sorcery
Samarden Press has Flynn's Guide to Magic in Traveller has a different magic system, magic item creation and careers
An example of a fantasy campaign
Terra/Sol Games has Netherell: Planet of Magic, with its own system of magic, magic careers, monsters and so on. It had a new way addressing background and education skills as well
One thing I found of particular use was the Influence stat found in Judge Dredd. I find it to be more useful in modelling D20 SRD powers where the force of personality was involved rather than Social Standing
Speaking of which,
Book 8 - Dilletante has the concept of scale with regards to Social Standing. I found that to be useful when addressing planet bound nobles vs multiplane nobles vs deities...