I think the flash card space is interesting because I think Anki users are very much "power users". Because it takes a curve to learn how to use it (poor UI) and there are lots of extensions, the type of person who ends up using it is a more advanced user.
The rate limiting is a big issue and I think something can be overcome. I'm working on my own app and what I've done is made it so you can limit the number of cards you review per day. Your daily lessons will usually be some combination of overdue cards and new cards. However, if you have too many overdue cards, there's no point in adding new cards to your lesson.
I know with SRS reviewing a card "just before you are going to forget it" is a big part of why people believe it's effective. However, I think that's just an imperfect model, so I think if you push an overdue card by a day or two, it'll still be effective. (i.e. there's no reason to force someone to review cards on a given day, obviously there's a window here where if you review too far in the future, you're going to find it harder) With this in mind, I made it so if you rate limited your daily lesson, the app just pushes the overflow to the next day. I think there are also other variables that make the models imperfect (some facts are easier to remember than others, how you present the content might make it easier/harder to remember, and so on).
Regarding easy input, I'm not familiar with how easy or hard it is to input images and audio, but I made sure that my app allowed a super easy drag and drop. In general, the app should make it incredibly easy to create new cards, edit existing, and add various media. Cards are the lifeblood of the tool, so working with them should be super fluid.
As for other modalities, I think that's an interesting topic to dive into. With self-learning applications, you have to balance making the most general use cases easy. Once you started adding more complex card types, your UI is going to have add affordances to make those possible, which has the effect of making it more challenging for the novice (who just uses text, images, and audio) to use.
Anyway, to sum up, it's definitely an interesting space because you have power users who may want to pull your app one way and regular users who may pull the app another way. I think finding the right balance of features is a challenge.