Bandit - Bouncy ball: Change the % Bounce in the spawn settings menu and spawn a dodecagon. Rotating items: Already possible. Goatsim - No. Read the FaQ.
Sour - Yes, this is a bit of a late reply, but that's because all items that are spawned go on top of the layering. This means they'll be visible if you put them in the barrel/cardboard. There are 3 ways to fix it - Spawn the barrel last, respawn the barrel (e.g. resize it), or use the detail tool and click the item inside, then click move behind.
Trung - Nothing. Not everything has a functionality. It's just for decoration. (Or you can make it sharp and link it to a cloner/cloner temp and make your own gun)
King - Sometimes the properties just don't work. Before making something, put ghost property on an object and see if it works. If not, refresh and repeat until it works.
The reason the barrel can't be grabbed is that it's made using "decorative layers" - Decorative layers basically don't exist and are only visual. You can't drag them, and items can't touch them, but they can be seen. The barrel is mostly made from those, so you can put stuff in it but can only drag it by the edges. (This won't be a problem since 0rava says he's changing the model, but still putting this out there)
Benji - You can. Go to "Properties" and add what you want. (For example, add Sharp to the BB Gun.) It's best to add shapes like Crumblers, Long Beams, Squares, etc. without any properties on certain places to stop the user from hurting himself with them though. (If you use crumblers or breakables, add unbreakable property.)
a7xmanliles1 - I think reloading won't be done, though now that I look at it again the FaQ doesn't specifically say it won't be added. Door: Not hard to make it yourself. Wheels: Use dodecagons/sawblades/chakrams.
Rainbow - Go to settings. Both can be done. Bob - Would you rather not be able to crush people with clouds? (If so, change weight to 1. If you want them to float as well, turn on zerogravity, change weight to 0, pin them, or change speed/spin damps to 999.)
Ronin - Not just poison. Unbreakable dolls can be killed in any way except cutting. (To make a truly invulnerable doll, you need to link an off-screen blood bag to every body part of an unbreakable doll.)
0d3b030214410161180010180 It probably could be harder, but I just tried to make it look interesting without paying attention to what each body part allowed and didn't, then did what I could with the allowed weapons. I'll do something completely focused on difficulty later. (Reduce the time if it's too easy.)
TheSmith - Go to Imgur.com and upload it (do NOT publish it), then take the link and post it in the MaD Lab Items thread (forums). If it's good, it might be added.
 "Mutilate-a-Doll 2"){kind=icon}
 "Star Glaive"){kind=icon}