Traveller itself has already used three outside systems: Hero, GURPS, and D20, as well as GDW's House System (which used a d20), three varieties of 2d6 systems, and two varieties of "xd6" systems.
One could argue that the old Ringworld RPG was a surrogate for Traveller using BRP (the percentile system that runs Runequest and Call of Cthulhu), though it is long out of print.
Similarly, Ashen Stars is basically Gumshoe Traveller.
There are fan adaptions for Fate (Spirit of Traveller) and *World (aka "Powered by the Apocalypse")