If you spent a day writting a cool new version of the "tabnew" Vim command, you'll probably want it to be the default command used to open new tabs. Right?
Luckily, there's an easy way to replace built in commands with custom ones: cabbrev. cabbrev will do a textual replacement, so if you add "cabbrev tabnew TabNew" to your vimrc, eachtime you type "tabnew" it will be translated to TabNew.
Bonus tip: The command is actually, "abbrev", not "cabbrev". The "c" stands for command: it's telling Vim that you want the abbrev command applied in command mode. You can also use it as "nabbrev" to have it applied in normal mode. "abbrev" it's a nice way to correct common typos!