Hi Félix,
- The QCPData structure and thus QCPGraph will lose its error bar capability. In this way its memory footprint will be strongly reduced for most use cases (most people don't use error bars). The error bars will be promoted to be an own new plottable, which can then be attached to more than just a QCPGraph. You could think of attaching error bars to the QCPBars plottable for example. So this is a win-win, it reduces the default QCPGraph footprint and at the same time improves error bar flexibility.
- There is no fixed schedule for QCP2.0 and I'm somewhat reluctant to announce a date officially. It puts unnecessary pressure on the development and I think that's what's wrong with many software projects. With QCP development I'd rather take longer, think harder, and then eventually make better decisions. What I can say is that there is at least another minor release 1.4 in the pipeline. If I find features that are currently planned for 2.0 and I notice during development that they could in principle work without breaking backward compatibility, another minor release 1.5 will be put together. So the 2.0 is still some time out, let's say 12 Months roughly (This is not an official announcement ;).
I believe you'll be fine if you modify QCPGraph (or a copy of it) with the changes you describe. Once QCP2.0 is out, you basically have two choices: stick with your QCP 1.x version and keep your slim QCPGraph-derivative, or switch to QCP2 and use the slim QCPGraph that will be shipped with it. And both choices will allow you to retain the low memory footprint.
Regarding non-trivial contributions it's a bit difficult. Since I use dual-licensing and reserve the right to provide other licenses, I am obligated to have the copyright on the source code. This means contributors need to agree to a so-called CLA, Contributor License Agreement, as many free software projects use. That makes sure the copyright on the contribution is transferred. Of course, non-trivial contribtors will be mentioned on the website if they like, but I can't have the copyright be spread out among multiple people. This is an unfortunate situation shared by many other free software projects. I don't like the concept of copyright transferral myself, and I wish there was a simpler solution.