What you request, seems a lot like siege mode but without running away with the flag. Maybe you mean like that:
Two teams, each has their countdown timer set in the beginning. One is unarmed, another is. Before armed team eliminates unarmed team, unarmed team's countdown goes down. And then swap.
First to reach zero wins.
Like:
Beginning: team A - 10min, team B - 10min
Team A: armed for the round, team B: unarmed
It took 2 mins to eliminate whole team B. So at the end team A. So timer A: 10min, timer B: 8min.
Now swap - team A unarmed, team B armed
It took 3 mins to eliminate whole team A. So: timer A: 7 min, timer B: 8 min
And so on, until one team reaches zero.
I like that idea. Much like "Last man standing" mode.