Time
"Do you feel like time is running out for us?"
The Doctor was lying in a field full of red grass which reminded her so much of Gallifrey. The Master was sat next to her, leaning back on his hands and looking up at the burning blue sun. He didn't answer straight away, continuing to look up at the sun. The Doctor sat up and looked up at the sun as well, trying to find out what was so interesting about it.
"Yes," He finally replies and they turn to look at each other and suddenly everything seemed to fade into the background; there was no knocking to worry about, there were no drums to worry about and the threats to the universe seemed to disappear.
Both Time Lords leaned in at the same time and eyes closed and lips brushed and this was how she wanted it to stay but eventually time caught up with them and they were on the TARDIS once more.
They never did mention that kiss, or any of the others, even after time had run out and the Doctor was running around in a new body.