For example, there were 31 episodes between Q-Who and BOBW.
Why not have the Borg appear again in force exactly 31 episodes after BOBW?
And so on and so on.
The ominous part is the "countdown" where fans know the Borg will return (and eventually the crew sees the pattern as well) and we see tension rise as we wonder just what new way the crew would come up with to defeat them this time.
I LOVE the prime number angle, it's like how cicadas have huge birth years every prime number of years (dif. for dif. varieties IIRC) so predators cannot time their own population spikes to eat 'em all. It beats having a Borg episode say, every seven of nine episodes or so. Sorry.