Chris Chibnall's run on Doctor Who has been generally seen as mostly unsuccessful, though it has its fans. Though Davies' run (seasons 1-4 of the revival era plus a year of specials) isn't flawless, he excels at writing characters and his return could see Doctor Who return to its more grounded roots.
Russell T Davies is the man who brought Doctor Who back from the dead with its revival series in 2005. His Doctors, Christopher Eccleston and David Tennant (the 9th and 10th Doctors), are some of the most beloved in the series. Davies' stories often focus on the interpersonal issues that stem from the complexities of travelling with the Doctor. Rose Tyler (Bille Piper) often finds herself trying to balance her home life with being the Doctor's companion, and David Tennant's Doctor is regarded as the most human iteration of the Time Lord yet.
The return of Davies marks a major turning point for Doctor Who as a series. His strength as a writer has always been his ability to write compelling drama and excellent characters, something that has been absent from the newer seasons. Though he arguably lacks Chibnall's sense of ambition and Moffat's capacity for genius ideas and amazing one-off stories, he's the best all-around showrunner the revival series has had yet. Many viewers take issue with the lack of depth to the 13th Doctor, which is disappointing given Jodie Whittaker's talent as a performer and her status as Doctor Who's first female Doctor. Davies can redeem this rocky start by writing the 14th Doctor more consistently.
Davies also has a talent for writing compelling side characters. Rose Tyler, Martha Jones, and Donna Noble's families all become major players in the Whoniverse throughout his run, which gives the world a sense of grounded reality, as the characters raise real-world concerns about their daughters travelling with a stranger claiming to be an alien. A return to this level of melodramatic realism is long overdue, as it was never a strength of Steven Moffat or Chris Chibnall. This kind of human drama will be essential in helping Davies' second run on Doctor Who stand out.
Davies is also unique for crafting a shared universe with Torchwood and The Sarah Jane Adventures, creating a sense of community and the biggest ensemble cast ever seen in Doctor Who. Though Chibnall has already brought back Captain Jack Harkness (John Barrowman), Davies can create new characters with their own ing cast to interact with the new Doctor and their companions. Davies gave the Doctor a pseudo-family for the first time since the Time War, and though there are plenty of plot points to pick up post-Chibnall, crafting realistic relationships with their friends and family should be Davies' first priority when writing a compelling 14th Doctor.
The future is bright for Doctor Who with Davies returning to helm the TARDIS. It's reassuring to know that Doctor Who will be taken back to Earth after Chibnall's ambitious-yet-flawed run on the series. Russell T Davies' best screenwriting trick is his knack for good characters and grounded conflict, which is just what the series needs to survive.