But yes, a good majority of the aliens and even certain characters in Doctor Who are owned by the writers who created them, which is why you often see a special notation in the episode credits about this race or character created by whoever.
During the Wilderness Years, there would be semi-professional fanfilms released, which would include iconic characters & aliens because they made a separate deal with the original creators but didn't have any outright references to the Doctor or the TARDIS so as not to infringe on the BBC. They even sometimes got legit
Doctor Who writers like Terrance Dicks to write them.
One of the more impressive examples is "Downtime," a 1995 sequel to "The Web of Fear." They brought back the Brigadier, Sarah Jane, Victoria, & Professor Travers, all played by their original actors along with supporting performances by John Leeson & Geoffrey Beevers as new characters. It was also the first appearance of Kate Stewart, long before she made her debut on the series proper in 2012, although she was played by a different (but similar looking) actress. And it was directed by Christopher Barry, who directed several
Doctor Who episodes from 1963-1979.
There was also a fanfilm called "Shakedown," which featured the Sontarans. But although Robert Holmes' estate gave them permission to use the Sontarans, the design of the Sontarans is owned by the BBC. They had to redesign the Sontarans from scratch using only the stage descriptions used in Robert Holmes' original script for "The Time Warrior."
Or maybe I'm just irritated that Whittaker's Doctor has such bad representation in the tie-ins with just four novels, and both her comic runs in DWM and from Titan seem to be on hiatus. Then again representation of the current Doctor in tie-ins seems to be in decline since Tennant left, with Smith not getting as much as him, Capaldi not getting as much as Smith and now Whittaker getting less than Capaldi. I guess Moffat and Chibnall just aren't as into tie-ins as RTD was.
I mostly got the sense that RTD was very protective of the new series as being THE
Doctor Who at the time, which was why all of the tie-ins around that era only ever featured the current Doctor, except for the Big Finish audios, which weren't even allowed to reference any elements from the new series. I suspect that Moffatt & Chibnall both have a more laissez faire attitude towards the whole thing.
It seemed like they kept the focus on the current Doctor during the first couple Matt Smith seasons. Even during Smith's later years, it seemed like the only non-Smith novels coming out were 50th anniversary reprints and a couple new books by Stephen Baxter & Alastair Reynolds, who are such high profile authors that they were probably given special dispensation to write for whichever Doctors they wanted. (Baxter wrote "The Wheel of Ice" with the 2nd Doctor, Jamie, & Zoe. Reynolds wrote "Harvest of Time" with the 3rd Doctor, Jo, UNIT, & the Master.) But those were the exceptions. It seemed to me that things didn't start falling apart until the Capaldi years, when the 9th, 10th, 11th, & 12th Doctors were all allowed to have their own ongoing comics series at the same time, along with various miniseries featuring other previous Doctors. During the Whitaker years, things seem to be mostly continuing as they were during the Capaldi years, although it looks like the ongoing previous Doctor comics have been largely discontinued.
Frankly, it seems like the only major change that Chibnall has made is that now all merchandise features the Whittaker logo. Before, it seemed that usually the current Doctor would have the current series logo but anything with the previous Doctors would use the old Pertwee/McGann logo. But now the Whittaker logo is on everything, which is weird!
That would be amazing! I wonder though if there might be some sorts of rights issue (just speculating, could very well be wrong). So far the only novel adaptations Big Finish has done have been Virgin New Adventures. They did adapt some 4th Doctor comics from the 70's. It seems like they have never adapted anything published directly by the BBC.
Didn't they do an 8th Doctor audio with Fitz? He's a companion from the BBC books.
I definitely plan on getting the "Timelord Victorious" books, although it may take me a while to get to them because I'm in the midst of a 1200 page fantasy novel and I'm not a fast reader. I'm not big into comics though. And while I like the Big Finish audios, there are too many of them for me to keep up, so I usually don't bother.