The ballroom dances should be better for Will ( no great fan of his partner, seems to be no interest or chemistry there !!!).
Fiona is unlikely to improve as much as is needed, everybody else is well ahead of her now, so really it should be her and Brendan.
However, as much as Dennis seems to be a really nice guy his dancing makes me cringe........... so with a bit of luck it will be him ( sorry I am now hooked on the Brendan /Fiona saga...............)