Doctor Who: 8 Fixes For Series 13
6. Don't Introduce Any New Companions
To make the previous point easier, simply don't overcrowd the TARDIS. Three companions is a big stretch in Doctor Who, especially when all three need to be fleshed out at the same time, something neither of Chris Chibnall's series has managed to do. Usually when three people (not including The Doctor) are present in the TARDIS at any one time, it's usually because another character is passing through for only an episode or two.
If we are expected to care for and/or resonate with the companion(s), give them time to breathe, and make their stories something we can easily consume alongside the threat of the week. Jumping ship between companions throughout the series for only an episode at a time made for inconsistent stories we didn't care about. When Ryan and Graham leave the show after Christmas, are we really going to care? At least, comparatively to how other companions have gone out?