I think the assault rifle for soldier idea is good, but it does ruin the whole point of tf2, so make like, a gun made by the bones of his enemies, so like every time you kill someone with the assault rifle, it gets a little stronger/or you get more health like with the eyelander taking heads.
As for the boomerang i think its a good idea. I think it would be better to replace the machete and the secondary fire is were you throw it like with the sandman.
And now for some class extras I just now thought of: for scout, same general idea as the scout, but as said before the scout will have a generic smg, the secondary a single use sensor that tells were enemies are near your about the distance round of a capture point. and instead of a bat they use a small knife kinda like the spys, but without back stabs and they attack faster. Thats all I can think of right now.