Open source would be great to see if it came to steam :) I just made a video of some of the things I want to see in Halo PC if it came to steam. Check it out here if you're interested:
https://www.youtube.com/watch?v=dnT4dphmbGQ<\a> But regardless I think it'd be a welcome edition if it came to steam. Just like MatthewDratt said plenty of people would likely repurchase it on steam and give it another go. People don't really play it anymore because it's not digitally available. If your game isn't available, then new players aren't going to be able to buy it or play it, and likely older players are not going to go through the effort to reinstall it and play it again. Plus there's no friends list or anything like that that would make the game social and want to played with other people. Having it on steam would bring all these things and would likely revitalize the Halo PC community especially if it got friends list and workshop support.