Yeah, it would be awesome to have this open-source, with the help of the community one can achieve great things.
There was this project for FirefoxOS to support J2ME apps -- https://github.com/mozilla/pluotsorbet It would be great to have something like that for "nostalgic" reasons. Many feature phones' users remember J2ME apps, games especially, very fondly. Check out also this J2ME project, which could potentially be ported to KaiOS as well: https://github.com/XerTheSquirrel/SquirrelJME
Also, have you considered some kind of support for "content blocking" (like in iOS, or in the form of typical browser plugin-ins), of course, I'm not saying that ANY content should be blocked by default. But taking into account that devices with KaiOS wouldn't be very powerful, it could be really useful to have such an option to avoid "unnecessary" stuff when browsing the Internet...
Anyway, keep up good work!