A custom gamemode where killing players switches them to your team.
The round ends when all players end up in one team, after which the teams scramble.

Easy to install, somewhat flexible and designed to co-exist with other vscript creations on the same map, which is far from a given. Should also be compatible with scramble plugins.

1)Put red_rover.nut into tf/custom/<any_name>/scripts/vscripts/.
2)Add logic_script entity into your map with the following properties

Entity Script...........................red_rover.nut
Script think function..........Think

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This is it! (all values are case-sensitive).

It's recommended to set a short respawn time - the script doesn't do that for you, and trying to force-respawn a dead player will make them A-pose even with a delay. Not sure why.