The best 24 hour contest is one that has always been
a 24 hour contest. Don't shorten existing ones. If
you must provide for part-timers (like me these days),
a 24 hour category would be fine. But don't make it
24 contiguous hours - that would be bad for the limited
time contester (who often has several fixed commitments
during the contest period) and also bad for the contest (which
would likely see major droughts from specific areas of
the world as the part-timers there tended to cluster
into the most "reasonable" 24-hour contiguous slots).
Remember the N6OP SCOPE proposal ("Sane Contest OPerating
Experience") for a 12 hour SS competition? He did it on
his own, outside the sponsor's rules, and a few of us
had fun competing privately.