5. There Are Too Many Earth-Based Story Lines/Companions
One of the cleverest things about Doctor Who is having a companion acting as our window into the Doctor's world: it allows the Doctor to be as alien as he likes because we, the audience, have someone we can relate to. It was the case when Ian and Barbara stumbled into the Tardis back in 1963 and it was even more prevalent in Nu Who, where modern companions Rose, Martha, Donna, Amy, Rory and Clara have allowed us to experience all of time and space from an early 21st century perspective. Of course, by having a companion with ties to modern day Earth means there is always going to be a number of modern-Earth based stories, which is no problem as long as it is tempered with a healthy dose of historic episode, future based adventures and trips to far flung planets, which the show has done more and more of in recent years. Budgets are always going to dictate that setting an episode on modern day will be infinitely cheaper. There are many fantastic 'modern day stories; Blink, Doomsday and The Eleventh Hour in Nu Who. In the Classic era the majority of Jon Pertwee's era was confined solely to Earth and it was during this time that we got some all-time classics; Spearhead From Space, The Silurians, Inferno, The Daemons, The Sea Devils... The fact is, like any episode, modern-Earth based, historic or set on another planet, it's all about having great script and performances, and modern companions work because they serve as our link to this world. Should there be historic, future or alien companions too? Absolutely - that's where multiple companions come in and it's something Doctor Who should definitely deliver in the future.