I can't see why they can't have c or d level subplots that could give something of an 'arc'. They could be self contained in each novel so that they don't 'put off' casual readers but still fit together to give some continuity and development between books...
That was the original
intention for TOS's so-called "lower decks" trilogies (2002-2008). It seemed like we were going to get the courtship of Angela Martine and Robert Tomlinson, but the next trilogy jumped past their interrupted wedding anyway. Then we got to follow the rise of some redshirts through the ranks, knowing that they'd probably get picked off across three, finally six, volumes.
The key to the first six books (The Janus Gate
, a trilogy by LA Graf; the Errand of Vengeance
trilogy by Kevin Ryan) was that they entwined the aired episodes and were intended, originally, to feature an ensemble cast of familiar faces: the "lower deck" characters, red-shirts, semi-regulars and guest star crewmembers of TOS. As I'd suspected (and later confirmed in "Voyages of the Imagination" by Jeff Ayres, 2006), these novel trilogies were inspired by a previous successful trilogy, "My Brother's Keeper" (1999) by Michael Jan Friedman.
Later, there was Ryan's sequel trilogy, Errand of Fury.