I believe that a lot of features mentioned above are good ideas, but most important is to keep it simple. For small developers is a very important development speed. Usually we create small innovative games, and most of predefined features will not help us a lot. I have the licenses of many engines that have all of these properties already developed, but problems arise with innovation, source code of those engines are difficult to change and takes a lot of time to implement new things. Productivity falls and a lot of teams dropping their projects because of financial problems.
C + + is great lenguage I am C + + programmer but in the indie world, and even some of AAA production studios often increase the use of language such as Python because it increases productivity because it simply shortens the development and up to 400 percent . Some off Us Are not good artists but in other hand some of us are not good C++ programmer . Just make it simple