Ashley 
Tom - I think the build service is going to be fantastic if it works without too many crazy hoops to jump through. As 
NotionGames has said, being able to feel confident to be able to deploy your games properly when they are complete is very important to instill confidence in your subscribers.
The upcoming poll voting system for future features sounds like a great idea too so that you get a clearer idea of what's a priority from the community.
A roadmap as other have suggested of upcoming C3 Features will help people get on board with paying for a subscription and let them know that they are contributing to the future of scirra. A lot of people feel like C3 (right now) is just an online version of C2 with bits of future C3 information spread across different threads etc. The blog posts help, but having all of the plans in one easy to see location would be much easier to refer to for everyone.
A tool that converts C2 2 plugins into C3 compatible plugins. I've spoken to 
rexrainbow and he thinks that if he has re-write his hundreds of plugins that it will take him a year to do so. So many people rely on addons that if they have to wait a long time to be able to use them (or to import C2 projects in C3 that rely on them) that they won't subscribe to C3. I'd go so far as suggest you either give them free access to C3 or hire them on your team full time. Plugin creators like him and everyone else who extends C2 are so valuable you don't want them to drop contributing to Construct.
I've seen how far C2 has come since the first version, i can't wait to see how C3 progresses over time.