Board Thread:Support Requests - Designing Your Wikia/@comment-5907266-20141105053641/@comment-4018164-20141106054417

I'm not sure where i saw this implemented for the first time, but if you add 4 asterisks to the dropdown, you can add as much cells as you want. For example (this also works in mobile skin): Though remember that the more cells you add, the more vertical space it takes, and users with small screens might need to scroll in the page for a proper navigation, so it's recommended to stay as close to 10 as you can.
 * Some ordinary tab
 * Blah 1
 * Blah 2
 * Test lvl1 tab
 * Test subject for dropdown
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test
 * Test

Another option is to do something like what ExtendedNavigation offers- to store some of the list items in "deeper" dropdown levels.