Why Blackmagic Design's Fusion?
Because, as Natron continues to develop at this pace, and as the project seemingly has had its sight on Nuke - as expensive as it is - all along and continues to do so, it is only a matter of time before Natron becomes a respectable contender in the node-based compositing marketplace. However, Natron should be cautious of settling for being a "free" alternative to the commercial industry standard, and look for other powerful, though lesser known, programs such as Blackmagic Design's Fusion (formerly Eyeon Fusion) because since BM's acquisition it offered a full functional version of Fusion absolutely free! with plans to to port it to OSX and Linux, as well. That means that Natron will lose its edge not only in price but also in cross-platform availability.
So what's Fusion has to offer that's missing in Natron?
Fusion is a powerful node-based compositing software yet it excels with at least two offerings:
1) A complete 3D (not 2.5D) workspace with camera motion
2) An excellent particles systems, creating very realistic fog, smoke, fire.. etc.
First, I know that Natron plans a 3D workspace, and I know how difficult it would be to develop this particular feature from scratch, so my suggestion is to utilize a readily made effort, namely the Gimpel3D software. Check it out:
http://www.gimpel3d.com/
As stated in the website, the developer had originally intended it to be a 2D/3D conversion software when it was such the craze in 2008 & 2009, but when he found that many of the functionalities were being offered by other programs, he decided to offer it as an SDK and later on openourced it and gave the whole thing away for free. The code has cool features for projection and camera motion in 3D space, in addition to projection mapping and depth painting, which I presume could be very useful to the Natron project.
Hope my suggestions help.
Good luck to you guys and keep it up :)