What do you mean by modding. Do you mean just UI stuff, or being able to add bosses and dungeons and stuff? If it's the later, not going to happen, EVER.

If your talking UI, with that comes the bad too.

You would see Bots with in game UI's, where the user interface for using the bot is all in the game. We would see automated crafting where you just select what you want to make and how many and tell it what steps to use and when and go afk. Come back and it's all done and your 10 levels higher.

Hackers would make a teleport hack (xyz) hack and give it an in game UI tied to the map so you just right click the map and click jump to, and bam your there.

I remember reading somewhere that SE said when the addon system comes out, they will have a submission process like an app store for phone apps, you make an addon, you submit it. If they approve it, they put it in the addon store and users can download it, otherwise, they cannot use it.