The reason we introduced the forum funding system is specifically to stop "bounties"...they don't work, and they result in sub-par product. Here's what we've observed with bounties:
Invariably there is a major disconnect between what the bounty-raisers think something costs, and what it actually costs. This can swing both ways, but it usually means that they put forward a lot less than it is worth.
The knock-on effect of (1) is that good software engineers, existing contributors, and freelance developers alike are all turned off at the prospect, as it doesn't appear to be palatable.
Because of (2) you end up with mediocre developers pushing out a barely passable solution just to get the bounty. Not only is their code / solution sub-par, but they don't stick around to maintain it afterwards, because they just wanted to get the bounty, not to create something they're proud of.
Ideas can be proposed by anyone, but before it goes to a funding stage it has to be picked up by someone who can do the work, and who is able to indicate how much it will cost.