Is there room for another language application?

The language learning market is a great place to launch a startup, but at the same time it is a very competitive market. There are billions of people in the world trying to learn a language and achieve a level of fluency that allows them to understand and communicate. And the truth is that achieving this level is quite ambitious. It is true that there are many applications and proposals to help in the task, but it is also true that there is room to find new solutions to accelerate a process that is in many cases frustrating and that many end up abandoning.

I would like to propose this topic for discussion for those who have an interest in this area. Some aspects that can be raised might include:

What are the most successful existing solutions?

Are there many different solutions for different types of people or are there some common features in the learning process that are worth investigating in order to design a wide-ranging solution?

What is the state of the art in the science of language learning?

I leave this here as an intro to see if it generates a useful conversation. It is a topic that I am passionate about and I have some personal ideas, and I would like to meet people interested in the topic and who are interested in developing a proposal in this area.