Charting out tectonic plates is a good next step. Some people start at there before creating the landmass but since you already have your coasts you may as well go to that step.
With techonic plates, you can determine mountain ranges by having an idea of what directions the plates are moving. (plates moving together tend to generate mountains/islands)
A combination of landmass shapes and mountains can help you determine vegetation and rainfall patterns.
These will help you determine where rivers should go.
With that all that you can start to predict areas that would have a lot of settlement.
There is a great tutorial around here somewhere about the process I'll try to find for you.
For your landmasses, I would recommend making the three in the middle have more varied sizes and relative distances from each other but I think your coastlines and land forms are otherwise very good and dynamic.