Hey, me again.
I understand your frustration. The answer to your question, however, is simply that no such list exists. At least, nothing updated. I think the last one that was attempted was Fenix's "Hot Debates" thread, but even that wasn't what you are looking for, per se.
You have to understand, this isn't a fledgling newcomer game that people are keen to fix up. Over the past 5 years, there have been tens of thousands of ideas. Several times, players have attempted to organize these ideas, and it just never stuck, mostly because it wasn't kept up-to-date. Couple that with a baffling forum policy about stickied threads enforced by our 'good-ol'boy' network of player mods, and, yeah, most of this stuff is just lost to time.
More importantly, there is almost no development of this game anymore. As of Rarebit's departure, we have been told by the some-time producer of MxO that there will not be any more updates (with the likely exception to turn on and off seasonal events). That's a really sore spot for a lot of us, and will likely bring some heat on me for even saying it. But from what I understand, even if this game were to develop a major glitch it would likely go unrepaired unless it rendered the game unplayable.
So, if you want to proceed regardless, and find out if something has been discussed before, just do a search. I recommend searching for your idea first in thread titles only. Then, you can search for the idea in posts by Rarebit, 9mmfu (NoRepro), Dracomet, HCFrog, and Walrus. That way, you get posts from Devs with their comments about the idea.
Or. . .you can just go ahead and post your ideas here. But, as I said in your other thread, be forewarned. There are some ideas that people have for this game that have already been hashed, rehashed, debated, argued, satirized, beaten until dead, beaten some more, beaten the stick that did the bea... well, you get the idea. Posting one of these ideas will almost certainly get you flamed by some people here.