Realism, Canon, Balance
There are all seemingly legitimate reasons, but they contradict all the time.
New things = Good
If the new thingy is super powerful then making it super rare balances out the scales.
If its below/at average don't just toss it out, it builds atmosphere.
Redundancy isn't bad, especially when it makes the game feel bigger.
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as for ideas I think most people ought to start learning, or even developing their own ideas if they can. If it was easy to know how to add things to reloaded source and test them on a private server or fake server, I'd probably like adding/testing new ideas out. Animations are the biggest issue about the game though, simply aren't the assets of nice looking weapons. Adding a sprite and using pre-existing animations is probably a thing though. Too bad not everyone has the free time to do all that xD!