Regarding 3D, I am not that interested in a full blown 3D engine but it would be very interesting to be able to draw a 3D box at least and set textures on each side and make it so it is easy to mix with the 2D engine. It could be useful for games like GTA 1-2 for example and I know it is not common use case, this is why so far I've found only SFML that support something like this out of the box, no other 2D framework and engine support it that I tried. If we could import and render OBJ that would be nice too, but that's it, I don't personally wish for anything else atm not even animations really or fancy shadows and lights, it would be fine if the 3D object could cast a 2D shadow similar to a 2D object and that's it.
Regarding monthly subscription for access, I don't think it could work with an Indie tool like Cerberus simply because most people can not afford it especially if you have a small user base to start with and you don't really have any extra services to offer but you charge for access only. What seems to me more logical for small businesses is a combination of subscription and a one time fee. This is what currently ClickTeam is trying and it is seems very reasonable to me.
The core product is one time fee, export platforms, plugins, also one time fee. But you can pay a monthly fee on Patreon and get extra content each month like plugins, extensions, DLC's, assets, examples, templates, tutorials also exclusive content that nowhere else can be purchased and you get to keep them even after you cancel your subscription. I believe ClickTeam also offer customer support on a dedicated forum or discord channel only to subscribers.
I think it is reasonable model, additionally I think would be also fine to release a paid version with a few new features every 6-12 months maybe.