Do novels come from author's own ideas/proposals, or are you ever commissioned to write a specific novel?
Short answer:
a bit of both.
Longer answers (specific to my experiences):
Day of the Vipers - Marco Palmieri liked my short stories in
Distant Shores and
The Sky's the Limit, and he had a
DS9 prequel duology in play he wanted to turn into a trilogy. So my brief was pretty much "write a political thriller that details the Cardassian invasion of Bajor and dovetails with the other two novels".
Seeds of Dissent - Marco invited me to pitch a novella for the
Myriad Universes 'What If...?'-type anthology, I offered three ideas and he picked the one he liked.
Vanguard - I pitched a novel idea for this series, but Marco passed on it and instead invited me to pitch for the
Titan series instead, which led to...
Synthesis - Was invited to write a
Titan novel, had a completely free hand on creating the story.
Cast No Shadow - invited to pitch anything
Trek for any era, I offered a story a post-
ST VI story about Valeris.
The Stuff of Dreams - Margaret Clark specifically asked for a
TNG novella set on the
Enterprise.
The Poisoned Chalice - was developing a stand-alone
Titan pitch when
The Fall miniseries came along, was asked to write the
Titan installment; came up with my story but collaborated with the other writers to make everything synch up.
Sight Unseen - had the stand-alone pitch mentioned above plus a TOS story idea; Margaret wanted a "24th century"
Trek novel for her schedule at the time, so I wrote the
Titan novel first...
The Latter Fire - ...followed by the TOS novel a year later.
Fear Itself - Kirsten Beyer asked Margaret to bring me on for a
Discovery novel, and the brief was "write a Saru story set before season 1, on the USS
Shenzhou". Originally it was conceived as an origin story for Saru, but that changed during development.
The Dark Veil - Once again, Kirsten asked for me to write something for
Picard; we originally developed a backstory idea for Seven of Nine, but that was eventually nixed. The brief changed to "write about Riker, Troi and their son Thad on the
Titan and/or Worf on the
Enterprise-E" and I chose to go for the
Titan option.
The Ashes of Tomorrow - Margaret approached myself, Dayton Ward and David Mack with the basic idea for what would become the
Coda trilogy, but we were given a free hand on the execution of the idea.
Anybody familiar with a writer's block?
Never believed in it. Everyone has good days and bad days in their job, but I've never liked the way the term "writer's block" mythologizes that part (or lack thereof!) of the creative process.