I'll do a basic breakdown on some of the positives and benefits of these. These are all just my perspective and personal opinion.
Gamemodes:
Currently the game modes in-game are fine. It offers people who are looking for an additional challenge the potential to do so. I personally wouldn't mess with the drop rates either, it keeps things fair for all players.
Skilling System:
The premise of offering alternative methods of making money to skillers sounds great. Because of the Grand Exchange's 250k rate it can be quite difficult for skillers to make money without Crafting or Thieving. An alternative method would be beneficial!
Grand Exchange:
Some of the items on the Grand Exchange could be beneficial to have removed. This would be basic supplies that will generate the economy such as Bones, Herbs and a vast quantity of skilling items.
Duo Slayer:
I am 100% for this idea, would be great to do multi Slayer!
Double Exp:
I like the premise of double experience, although I would recommend saving it for special occasions to make it more of a special event.