Oh, I can think of a few!
- H&K MP7 clone in 17HMR - it would pretty much be a legal version of the MP7 with a legal ammunition and roughly the same performance. That would be neat.
- A modern 45-70 semi automatic rifle with a 20 round magazine in a bullpup design that works with really heavy bullet weights like 500 grains and up (Yes its a rimmed round but the SVT-40 in 7.62x54r works just fine too)
.500S&W would work too.
- Glock in 7.62x25
- Beefed up heavy barrel Steyr AUG in .308 that takes AR10 mags