I attended a talk on illustrating early chapter books at a SCBWI conference a few weeks back, here are some of my notes. In the talk, the art director said there were two "categories" for early chapter books
First to Second Grade These manuscripts are normally 4-5 pages of plain, typed-out, unformatted text. There are illustrations on every page of the final book layout and they should relate very closely if not exactly to what is written on the page (kids will use them as context clues). This normally results in a 60-80 page book.
First to Fourth Grade These books can be slightly longer, around 90-120 pages and have illustrations on most but not all pages. The illustrations have a little bit more flexibility as to how closely they align with the text.
As far as genre, she was saying that almost all beginning chapter books are simple concepts, exciting, action packed, and do not try to tackle any sort of complex emotional issues. Many of them are silly or include some sort of comic relief. They are meant to help kids improve reading skills and feel excited and accomplished about reading on their own, not get them thinking about plot or theme. More complex plots or challenging themes are more appropriate for middle grade chapter books.
If you were going to try to publish the traditional route I think you'd want to write the manuscript and then shop around for a literary agent to sell it to a pub. You could mention that you are interested in illustrating and do some samples but the publishing house's design team will have a lot to say about word count per page, page count, where illustrations should go and how much space there is for them.
Another thing that I noticed was that they make almost all early chapter books into multi-book series (like magic treehouse). That may be something that you'd want to consider. Maybe you'd want to shop around a manuscript plus ideas for the next two books instead of just the one. Good luck and I hope this helps, I think that your mouse adventures could make an awesome early chapter book!