I'm sorry, but I'm not sure I understand.

1. You have a working app that people like.

2. You have a set of new features that are being requested by some users.

Seems to me that the obvious solution is to bundle all of these new features, create a "premium account", and charge a monthly fee to those who wish to upgrade.

Voila!

The problem is that many features are going to cost more than one user could reasonably donate, and a lot of others are interrelated.

If you're set on this idea, I'd recommend looking into the Street Performer Protocol, which basically says that people pay for feature X.. If it's not implemented, they all get their money back.

http://en.wikipedia.org/wiki/Street_Performer_Protocol

I meant allow all the users to contribute to creating a single feature. If you had 50k users and needed 5g 10% of users only have to give $10 to reach the goal.

SPP is the right way to do that.

The problem comes in that users will look at it and say "What if only 200 of the 400 people donate? Then I wasted my money.."

SPP basically promises that they'll get the money back, if they don't reach the threshold. If only 200 people donate, then I'll refund everyone, and no harm done.

The other alternative is taking "Pledges" and only collecting once you have enough to make it worthwhile, but I recommend against this. The problem is that if it takes a while, people might forget they agreed, or might not come back, etc.

One final alternative is to collect their credit card information and pledge number, but only charge them if you hit the threshold for implementing the feature. There is then a risk they might forget and chargeback, or their card might expire.

I think the backpack model is the way to go. Everybody can get free access, but there are some arbitrary limits. Pay $5/mo or year and those limits go away. Also, there's no reason to disturb existing customers. Never never, take anything away that you've given people for free. If you feel your new feature isn't the thing to limit, be sure to roll your existing users into a special $0/month premium account. just ask new people to pay.

What would you do if you had users but they weren't requesting any new functionality? Do you have your own plans for what you want to add or would you start working on something else? Also, how do you know your users are happy?

how about you just make it and then try to make money off of it :)