First of all I must remind that Thomy is not currently developing and creating new content due to a temporary leave.
1. New arenas etc are always welcomed, but is it the wisest choice? I personally like the idea of Agility Pyramid, which would make it more fun and rewarding.
2. As I am going for 200m fletching right now, I would love it, but it would crash the prices on both flax and bowstrings. Collecting flax as a money-making method will no longer be viable. A bank near a decent flax field would be more appealing.
3. We already have farmers and master farmers for that. Yes, they move, but it would still be faster than stalls due to stock respawn times. I wouldn't mind this, but it's not really needed.
4. Yes, community has wanted them for a long time. With this new cosmetic items etc should be introduced.
5. I have always liked random events, especially the ones with rewards like emotes and cosmetic equipment.
6. Cooking guild does not work at all. You can't go to the upper floors. Also cooking is based on cooking fish and meat currently. You can also make bread, but that's it. No cakes, pizzas, pies etc. Currently it doesn't need a bank. If more cookable items are introduced, why not.
7. Crafting doesn't need new items. If they were to be added, my suggestion would be to use runecrafting for them (ofc you have to make soft clay for the base, currently not possible either). I like the current version of teletabs, they're fairly easy to get, but can still be sold for ~10k ea if requested, which makes it a decent moneymaking method (this should stay even if making tabs is added). Also in rs making teletabs is construction (player house) based action, so this may be saved for later when (if) construction is released.
8. Can be gathered via fishing and pvming (dagannoths, werewolves, goraks). Not needed. Snape grass and prayer potions are over-rated anyway.
9. No! The infinite run energy is a great addition to members. I really disliked the run energy management in rs, so this is great. However rest option could be added, so non-members would have a way to recharge it. Also we already have agile set and boots of lightness, which decrease weight, so graceful isn't needed (it's a good reward for any agility minigame etc though).
10. I would like to see more quests and I've even thought about creating some using Emps-Script, but original stuff isn't that easy to create. Personally I would like to see all (as much as possible) runescape quests to be added, since they are fun and also create more content. Yes it's one-time only, but still a nice addition to the game. I know that for some reason Thomy doesn't want these to be added - he wants original content, but I just don't agree with his views on this.
As far as your suggestion about quests is concerned, I do not agree (for now atleast). We already have some items and areas which can't be used/accessed otherwise and for now we don't need more. However, if a quests adds a new region/a new monster/gives a new weapon I'd agree.
Btw look up the word "prey", because it's definitely not what you meant