Almost. Here are some suggestions, but it depends on where you are starting from....
1.a. Already have an open worksheet? Click SQL icon in worksheet's toolbar or use Ctrl-Shft-N to open an (unshared) worksheet.
1.b. Already have an open worksheet? Drag and drop an existing sql file.
2. Nothing open? Click SQL icon's drop-down arrow in main toolbar and select a connection.
3. Nothing open and in the Connections view? Click on a connection. A new worksheet opens if ...
Tools > Preferences > Database > Worksheet > Open a Worksheet on connect
is checked off.
4. Ctrl-O and put your favorite sql file at the top of the list in the default directory the Open dialog displays.
Etc, etc ...
SQL Developer Team
Would you agree that a File-New option with corresponding toolbar button and keyboard shortcut would be quicker, simpler and friendlier?
I've got used to most of the idiosyncrasies now, but it takes a long time. It's great that you take the time to answer these questions on the forum, but it would be better if you could standardise and streamline the GUI a little more. Another example is the missing Copy command on the right-click menu (only in the data grid).