I like the idea. It would be nice to have accessories. Just to be specific, I take it, you mean static lenses, not zoomable ones. With kits, you mean DIY kits or a selection of ready to use lenses of different characteristics? And I take it, something like a lens camera is not part of this idea.
I’ve had several lens kits over the years. Some were veeery cheapo plastic lenses but they had a nifty magnetic ring with a sticker/adhesive side so that you could fix that to your phone and quickly change small lightweight lenses. Quality of lenses and pics varied but usability was good. Luckily the quality has improved over the decades. A good selection of useable ones is hard to come by. I’d like to select specifically which ones I’d get (usually very wide angle or super telescopic).
The various clips to attach the lenses allowed for bigger and better glass lenses - and longer lens structures that the actual phone - but those were not only heavier, it’s been difficult to place those precisely to get the focal point right. Some models have had available “corner cases” that fit perfectly to the corner of the phone and get the placement right. But those have been a bit bulkier (usually not an issue though) and hard to come by.
The attachment mechanism has varied and I haven’t come across with a good standardized variation - I’d really like there to be one or two that were popular in order to use many different lenses (and not to have a single supplier monopoly system), so using something that is already in use and usable would be great.
And it’s another thing to be able to have a a good one that can be used with a case (those printable ones since other options are scarce). It would be nice to have something that would work with or without a case (noting that focal point shifts when lens is closer/farther from the camera. Could there be a printable adapter for a leans (or even a carousel adapter that would allow for fast change of lenses)? Or an adapter that would allow using some already existing good lenses (or even those meant for real cameras)?
Other accessory ideas would be the “camera handle” thing that has been available for few models. It could bring together a few things. Basic idea is to have a better grip, additional battery, a standard connection point for tripod and to have that attachment for a lens.
I hope you can take into consideration even some of those.