A limbo bar/game seems to have many possibilities.

With a violet wand you could have an electrified bar so any parts that
get too close get zapped.

You could use a flaming limbo bar and depending on your preferences might
add a little alcohol/lighter fluid to the subs breasts/bra/top to help
encourage staying below the bar.

For those into fire games maybe a bit more alcohol/lighter fluid to
encourage lighting her breasts/bar.

For even more fun for those into extreme/edgy/offbeat fireplay you
could fasten a lit candle to her crotch so she has to light the bar as
she goes below it.  A short enough candle might encourage a faster trip
under the bar.