There are several that were ripe for coming back.
Botha - while the ending of "PERSISTENCE OF VISION" is great because it left more questions than answers, they were so mysterious that I would have loved to learn more about them.
Swarm species - so different that the universal translator couldn't really pin their language down. Their ships work as a full on collective. It's almost like the writers were trying to prepare us for the Borg later that season.
Voth - I would have loved to see them again. "DISTANT ORIGIN" is one of my favorites from the series. A deeper look at their culture would have been awesome.
Krenim - a species that uses time itself as a weapon? Genius, and feels like a DOCTOR WHO nod.
Chatic Space Dwellers - while the execution of "THE FIGHT" was bad, the concept was unique, and so was this alien.
Vaadwaur - they were supposed to be recurring. Even dialogue at the end indicated this. They were the conquerors who got beaten, and now everyone is more advanced than they are, but because they know the underspace so well, they have an edge to recover a lot of what was lost. This needed a followup.
Kobali - unique method of procreating. Though the bigger question no one raised during this episode... Ballard died 3 years before this episode. (It really should be 2 years, because that was the first encounter with the Hirogen, but we'll table that other inconsistency for later.) That's about 30,000 light years ago... resurrected Ballard caught up with Voyager pretty easily, eh? And with not knowing where exactly to find them. And her new father was right behind her. Yet no one asks about the method of travel, or examines her shuttle? Could have easily shaved the last 20,000 light years off the rest of the trip.