The phrase "a completely appropriate" is correct and usable in written English.
It can be used to describe something that is entirely suitable or fitting in a particular context.
Example: "The dress code for the event is formal, so wearing a tuxedo is a completely appropriate choice."
Alternatives: "entirely suitable" or "perfectly fitting".
To put it another way, color was being made out of harmony, and Bach (admittedly an unusually chromatic Bach) was finding a completely appropriate place alongside Liszt and the 20th century.
A cluster of private equity firms is backing startups in this area – and that, in contrast to the bottled drinking water business, strikes me as a completely appropriate quest to way to profit – not from water itself, but from the need to consume water more efficiently.
If the U.S. government cuts a trade deal with Vietnam that results in children making Nike shoes for slave wages, then people who buy Nikes will find out about it and throw a completely appropriate fit.
In a completely appropriate twist, Irvine and Hankins gave birth on Sunday in the same hospital within the same hour.
I need to engage in something physical that allows me to blow off some steam -- in a completely appropriate manner.
This guy wanted to pay tribute to Vonnegut and realized that getting a tattoo of my coaster was, in his words, a "completely appropriate and beautiful way" to do that.
And a good friend was in mourning when her son became engaged to a lovely, completely appropriate young woman, her only flaw being that she was not Jewish.
Because of these features, such an experience embodies a maximum of objectivity in the subject's relation to an object completely appropriate to its subjectivity (GW 17: 170).
There was a bite to the winds, a ghostly quality to the restrained strings; and this conductor's trademark pauses created a suspense that was completely appropriate to this music.
We think a bid test is completely appropriate.
Oh, and they think slurs against transgender people are completely appropriate in a show that co-opts a play/movie about the world's favorite fictional transvestite.
