I wouldn’t be interested in an app unless I can run it on my laptop. But that’s simply a device choice and not related to the general idea behind the app 
I like your idea connecting one onomatopoeia with different resources to learn from.
- As you already pointed out the item may be used in very different situations and having multiple examples to experience the item in context sounds like a great idea.
- As well as addressing different senses (hear / see / …).
- A feature like bunpro has with referring to different already existing resources that provide a deeper background on the item might be cool as well.
There are already a bunch of great resources out when it comes to learning specific onomatopoeia. So, when encountering an item in the wild I already have resources to go to and check the meaning.
What I wish for would be a resource where I can look up items the other way around. This means there is a certain situation I describe to the resource and it then gives me an ordered list with suitable items.
- E.g. Onomatopedia does this with the category system they applied for their entries. Depending on how many entries a category has I still have to go through a couple of items to find a suitable one.
Another thing I wish for, and which is what I am kind of doing with my current approach, is to learn items gradually. You already gave an example (ぶつぶつ) that may be used in very different situations. I would be still interest though to learn these different aspects about one item step by step and not all at once.
- To elaborate a little bit more on this aspect take the 生 kanji as an example. It has a lot of different readings and is associated with lots of different vocab words. They are related somehow but still, trying to learn all of the different meanings at once would simply overwhelm me. Therefore I prefer to start with one or two only. At some point further in my studies I will encounter examples when my limited knowledge doesn’t fit and gradually expand it.
Hope you can draw something useful from this feedback. Feel free to ignore everything else. All the best for your project 