Just to mention, there is no already planned features for GD. I mean, normally, we never know what is going to be implemented next and also no guarantee for that if there are any features or improvements you would like to see is going to be implemented in the future, ever 
But I think we went far away from the original idea, mainly we want GD to be released on steam as-is not to fund the development of any feature or new version.
I think 4ian need to decide first if he want the development of GD to be funded or only Steam Release.
If 4ian want only 100 euros to release GD on steam, he only need to mention “Linux” and “Game Development” and “Steam” and “Free”, in the first minutes 100 people from Linux and Steam community going to offer 1 euro at least, can’t be a problem. But if he want to gather even more money, in my opinion some mugs and early access wont be enough he need to offer something related to GD and exclusive, no other way around it.
The problem is that, GD is completely free, there is no early version, beta version, premium version\features nothing to offer that could really convince anybody to support. I mean mugs, t-shirts, your name in the special thanks section…etc only a nice way to express our gratitude and a nice little extra for support but not going to convince anybody. Need to offer something that people want. For such a free tool, the best offer is feature.
The vote on feature can be a good stretch goal in my opinion. Every cent above 100 euro, could be spent on to the development of a feature. Personally I liked the way as they did with 3DRad. Unfortunately it was not successful because the money gathered slowly and finally nothing implemented and the development of the project is stopped after all, but mainly a good idea, which is to make a list of features, set how much it cost to implement, and everybody who donate can choose which feature would like to be implemented. Similar to stretch goals, but the funder can choose in this case. I don’t know if it possible with any fundraising site, but I could imagine something like this to convince people to support GD not only to be released on steam, but also to implement features they would like to see in Steam version. In case if a feature is not funded, all it fund going to be spent on the next feature, or funders could choose more than 1 option by priority.
If it too complicated simply make traditional stretch goals:
-100 : steam release
-200 : this feature
-500: that feature
The other option is to make a premium version of GD, maybe the Steam version can have some premium extensions (funded features), not available for free but only on steam only for funders and people who buy it on release. And also beta access for funders.
I know it sounds horrible, but In my opinion GD can’t be free forever, at some point 4ian need to make money for living if not already. This is life, nothing can survive if it free, not supported and make no money, except if it open-source. So maybe this is the time to try make a premium version at least for extensions\features on Steam by offering premium features for funders and buyers. If you want to keep it “free”, sell it cheap on release, but some way the reward need to be related to GD and exclusive to funders\buyers.
But if you hate the ide of any premium content, then simply try to fundraise by saying you want to release this tool with this features, cost that much to be available for free for everybody and such.
I have seen FREE projects funded successfully but not many and most of them was free as “open-source”.
Anyway, all I want is a Steam version with Workshop 