An angsty take on the Master's drums - what if, when they were children, Koschei would lie down with his head on Theta's chest, lulled to sleep by the steady rhythm of their heart beats? The drums, the turning point for their relationship becoming strained, are a mockery of that once comforting sound because they'll never be that close ever again, and it drives the Master mad with grief and pain.
ive thought abt something in the same vein, but re: the headcanon that after the link with rassilon is broken, the drums stop in the master's head
because if the master did lose the sound of the drums, even though it drove them insane I think that they'd genuinely miss hearing them. like okay,, I couldn't tell you if missy would ever stoop so low as to ask the doctor to let her listen to his hearts but if they ever got close enough during the vault era for him to sleep around her (again, idk if this is LIKELY but work with me here) she would at least be tempted to put her ear to his chest, just so she can be lulled to sleep accompanied by the beating
I'd say same with the spy master but imagining 13 trusting him enough to sleep in his presence is pushing my imagination too far i fear