I wouldn't prioritize the most common words unless you're competing. Thing is, even with my many changes to my keyboard I want to be able to use other people's boards with only a moderate amount of annoyance.
You don't have to type in shorthand all the time, when you're on other computers you could type in full. Or bring a copy of Phrase Express with you. Or type in shorthand and use a script to convert it to longhand later on. You could type Colemak-shorthand and QWERTY-longhand. You could have an alternate spacebar key to expand abbreviations. You could use "tnh" for "Thích Nhất Hạnh". The choice is yours.
Bear in mind that shorthand has already been in use for centuries in some form. One problem with the current crop of optimised layouts is that they try to improve on an outdated system and ignore the advent of computing. Typing words out letter by letter made more sense in the age of typewriters before computers were around. Now though, we can leave behind an outdated system.
Keyboard shorthand is already in wide use by professionals who provide real time meaning-for-meaning transcription such as C-Print and Typewell providers. The younger generation uses it all the time with texting. If kids can do it anyone can.
T9-QWERTY - my port of T9 to the PC (a work in progress); T9-MOUSE - COMING SOON
Keyboard Shorthand