A DM homebrewing something for his game is one thing.
A Player homebrewing someting for someone elses game, with out consulting that person, is an elephant of a different color.

Everything you have listed so far (with the exception of the Int 16 Barbarian) are things that can alter game balance, if not though through very carefully (in respects to how it could possibly be used and/or interact with rules already in play), or, in the case if your "Magician", redundant.

But let's go back to the first two sentences....

Donny DM goes through a lot of time and sweat to creat a campaign, coming up with all sorts of details and situations. Then along comes Petey Player, who starts throwing out random "ideas" to changes/modify all that work, simply because "wouldn't it be neat-oh if".

Now, I can't speak for anybody else, but if a new player to my group did that..I wouldn't be very receptive either. Not just because it's a minor F.U. to the DM, but because all of the extra work a DM now has to do to make sure your "idea" isn't OP as hell, or conflicts with an existing rule, or is just a re-wording of somthing that already exists somewhere else with the possible consiquence of having all the benefits, but none of the draw backs (read OP) of the original thing, and then find a way to work all of this new content seamlessly into his/her campaign.

And letting YOU do this, opens the flood gates for every other player to try to homebrew his/her own stuff, because "he did it, why can't I?". And then the entire campaign de-rails while everybody is off being a Rules Tinker Gnome (TM).