actually i have always thought something that is under the radar to expand in america is a pro rugby league (is there one nowadays? i dk).
i always mention how crazy popular rugby is in the usa when i talk to brits and australians and they are quite surprized. the thing is, its a college club sport here. very niche, but super popular at that level. its prime for a pro league. well...it has been forever and nothing has happened, so maybe not???
edit -- i guess there is something more at a national level for rugby now --- founded in 2011:
https://en.wikipedia.org/wiki/USA_Rugby_League
that said, lacrosse has also certainly taken off. it too was mostly at a college club level but has grown to being played in many high schools. there is a pro league since 1999 where players make $10k-25k. with better pr that could also expand too i think:
https://en.wikipedia.org/wiki/Major_League_Lacrosse
both of these are great spectator sports, more so in person than on tv though.