I like the idea of Tegan taking over for the shouty bits.
The sad thing about the Four to Doomsday line-splitting s that it had a lasting, and arguably negative, effect on both characters, as it pushed them to extremes, and later writers picked up on that. Tegan became more aggressive than she would have been, and that got latched onto, and Nyssa became more passive and meek than she had been in Kepper of Traken.
I mean, just look at what Nyssa does in Traken, largely off her own bat:
She sneaks Adric into a crime scene that's been sealed off by the local police by a) telling the policeman that as her father is a consul, the law is whatever he says it is, and hence she's above it; and b) bribing him.
Then when her daddy's arrested, she sets out to mount a jailbreak. After finding that tactics a) and b) no longer work, she tries c) she guns the copper down with a workbench tool.
And finally, on learning that the Doctor's told Adric how to build a device that will destroy the age-old basis of her entire civilisation, she tells Adric to get on with building it, and then installs and uses it!!!
Now just imagine how all that would seem if she was from modern Earth: "Senator's Daughter BRIBED POLICE! Attacked Officers! Sabotaged Nuke Station!!! Shock Horror!!!!. Oh, hold on, I've just spoiled season three of Homeland. Sorry.