Solving the Problem
Is this enough to address the criticism that Amy and Clara were the central focus of Doctor Who a little too often? Very possibly, yes. In what has aired of season 10 so far, Bill has been a very entertaining figure and has demonstrated sparkling chemistry with the Twelfth Doctor, but has only ever been one component of the narrative. Even the character's debut episode - where you'd expect the focus to be solely on her - kept things relatively balanced, also introducing viewers to The Doctor's new living situation and setting up some key long term story arcs.
The ease and speed with which The Doctor and Bill have established their relationship has been beneficial too. Most modern companions require a bedding-in period, whereby both characters get used to each other and develop their own dynamic. In Bill's case, this was comfortably established by the halfway point of second episode, 'Smile' and both characters quickly settled into a natural groove.
All of this means that Doctor Who's tenth season has been given the freedom to focus on the individual stories contained within each episode - all four of which have been largely well-received to varying extents. Moreover, the ongoing 'what's in the vault' mystery is given top billing as this season's premier topic of discussion. And in the midst of this you have Bill, acting as an effortlessly watchable supporting character and bringing a welcome breath of fresh air with plenty of wit and charm - but no galactic baggage.
This also helps to alleviate another oft-cried complaint of Moffat-era Who: the complexity. Arcs such as The Eleventh Doctor marrying River Song to avoid his own death and Clara jumping into the Doctor's time stream were considered highly convoluted by some, and perhaps too clever for their own good. The simplicity of Bill's character has impacted on the season as a whole, and fans can tune in each week without feeling the need to feverishly take notes in order to keep up.
Unfortunately, there is potentially a disheartening reason as to why Bill is being kept away from the heart of Doctor Who's main narrative: that she's not sticking around for long. With Capaldi and Moffat's departures imminent, there are reports that Bill too will be making her exit at the end of the season in order to give new show-runner Chris Chibnall a fresh slate to work with. Hopefully such rumors prove to be false, as Bill Potts is currently proving to be not only a great companion but she's also brought a much-needed change of dynamic to the show and deserves more than just a single term in the TARDIS.
Doctor Who continues May 13th with 'Oxygen' on BBC and BBC America.