Magento dropdown menu change to multi-columns

I need to change dropdown menu of the Magento theme. There is already the dropdown menu, working well, but it's single-columned. The problem is that we have more than 50 subcategories under some top categories like Shop By Brand. With the current dropdown, only about 20 of them are able to be shown. Can never access the last 30 subcategories due to the screen size. So it must be a good idea to have multi columns to show many more subcategories. It will be like there are only first 15 subcategories on the first column and the next ones will continue on the second column and so on. For example there are 20 subcategories, we will have two columns – 15 on 1st column and 5 on 2nd column.