Well, it seems like an interesting project you guys (and gal(s)) are undertaking here.

I'm sorry if the answer is written somewhere, but exactly how do you envision the final engine?
What sort of features do you dream this engine will have in a stable state?
It's a dead given that it will offer 2D/3D rendering, (basic) input system and other basics for game development, but what about the higher level stuff?
What's the overall arcihtecture of of the engine and a game developed with it? What kind of game object/entity handling will be offered?
Will the engine be more data driven and designer friendly versus focussed on just exposing functionality through an API?
Will there be an editor at some point?
What kind of engines would you like to compare this with? How ambitious of an engine do you want to create?

You probably haven't settled on many of these things yet, but at this point all I know is that you want to make a game engine. What that means to you I don't know.