I don't want them to be scary. I think looming scary threats that last forever are un-Star Trek.
I think it was a GOOD THING that they aren't as threatening after Voyager. Heck, the fact that there was a Collective after the Hugh episode and Lore/Borg episodes took a retcon.
They should never be as threatening as they were at Wolf 359 EVER. If you like how that episode made you feel, I think it's better to demand new, scary stories with new species and let the Borg settle down into a more mundane life as a more mundane power.
At a certain point, Bane and Doomsday are just part of Batman and Superman's rogues galleries. At a certain point, the Shadows and the Vorlons and the First Ones ain't anything special and need to be told to go away. Buffy doesn't fight the same Big Bad twice. And the Master is just a scared and victimized bully underneath his bluster.
NOTHING should be perpetually scary past its fresh date, not even mindless forces of nature.