The SCons wiki has moved to https://github.com/SCons/scons/wiki/Proposal
How To Write a Good Proposal
This page outlines what is needed in a proposal that will be acceptable to SCons. For a source of potential tasks, look at the ideas page. You can also look at the mentors page for information about the people who will be doing the evaluations.
You don't necessarily have to do all these steps in exactly the order that they are given here, but some parts need to be done before others, so this is a reasonable sequence to do it.
Don't be intimidated. First and foremost, don't let the apparent complexity of this process scare you away. Preparing a proposal may look like a lot of unnecessary hard work, but if you want to be accepted (by <ins>any</ins> organization, not just SCons), you'll have to do it eventually in one way or another, so you may as well get it done and have it out of the way. Moreover, you'll find that it's not as difficult as you might think; if you know in your mind what you want to do, this process is just a matter of filling in the blanks. (And if it isn't clear in your mind, this process will help clarify it.)
Check the requirements. Make sure you meet the requirements below. If you can't meet the requirements, you will eventually be disqualified, no matter how good the proposal is. And look at the ground rules below as well to see if any of those might cause problems for your project.
Float your idea. If you aren't already subscribed, sign up to receive the dev@scons mailing list. Write a short note to the mailing list and tell us what you're thinking about doing. Or you can try directly contacting one of the mentors who you think might be interested in the topic, but realize that you'll probably need to discuss your proposal on the mailing list eventually, so you can save some time by starting there. If the response is favorable, go ahead and start preparing the proposal.