It would be a copyright issue, not a trademark issue. Copyright's kind of a cumulitive thing. The more you have in common with Cap, the more it becomes clear that you are just copying. Liefeld's character Diehard had a boomeranging shield, but Diehard was a much different character than Cap. Marvel didn't have an issue with Liefeld doing boomeranging shields until he gave one to Fighting American, which made him almost identical to Cap. The Golden Age MLJ character, The Hun threw a round shield with a swatsika, although it didn't really rebound. So I think you can use the idea, as long as the character isn't otherwise a Cap clone.