Lots of interesting ideas, but quite a number of recent suggestions seem to be computer games, which are surely best played on the computer.
I second all the references to classic scifi (Asimov, Clarke, Heinlein etc), but it strikes me that one of the beauties of Trav is most of these can be easily converted, perhaps with some modifications, e.g. Heinlein's "The Moon is a harsh mistress" would make a brilliant TL8/9 campaign, so perhaps not a need to publish supplements to cover them. Bit surprised to see only a few mentions of Hamilton's Night' Dawn trilogy (perhaps only a few people managed to get to the end!). I could see this as quite a high TL background with many interesting ideas, even without following the return of the dead storyline.
Very surprised to see no reference to Farscape, I know that the storylines rather lost their way once Crichton returned to earp, but the backgrond of competing empires, easy interstellar travel and a range of alien life seems tailor made for mid tech (TL12ish) traveller, and translater microbes solve so many long running communication problems!
Overall, I tend to think that unique, tailor made, background (3I or other) is the best way to go. Would parts of 3I history make good sourcebooks with adventures, e.g. the Interstellar Wars (how inevitable was the defeat of the Ziru Sirka?) ?