Regarding retention. I think some of the things Tom listed are good. Despite having been playing around with Construct for years I really enjoyed poking around the example games in C3 so more 'full' games in different genres is a good idea. I also think that more detailed, cohesive and professional tutorials are a must. The community tutorials are great, don't get me wrong and I want to see them continue and flourish. But many of them show very esoteric or downright inefficient ways of doing things and sometimes reading them it's like the blind leading the blind. Good quality tutorials from the devs on efficient and proper ways to do things I think will help Construct's image and bolster the overall quality of the community's output, which will in turn attract new users and retain current ones.
However I don't think these points will be enough to improve user retention and the reason is that hobbyist game devs are pretty transient. We hop between different software, trying new things, experimenting etc. All of those guys that impulse bought C2 aren't going to impulse subscribe to C3, because psychologically buying something and renting something are two different things. I've done a few team based game jams and really it depends on the team, but every time we've used different game making software. None of them have been subscription based, I don't think it would work unless everyone was already subscribed as subscriptions don't really fit into that flexible, spontaneous mentality. The monthly subscription thing might help with this, but it might harm you as well. It's quite a difficult situation.