The Mirror Universe
One of the more popular episodes of the fourth season was Enterprise's foray into the mirror universe, which originated with The Original Series' Mirror, Mirror, and more recently seen in Deep Space Nine. Whilst you could criticise In A Mirror, Darkly, for not being the best episode to introduce new viewers to the franchise as the episode required knowledge of the series in order to recognise the differences in the characterisation. Plus there was the Gorn as well. But let's not get into that, because at least the Tholians were good, so one out of two wasn't bad. But the intention was to return to that universe during season five, referred to as a mini-series within the season. Other than that, we don't have a great deal what else to deal with but the presumption is that it would follow Empress Hoshi as she secured her power base.
The Borg Queen
Despite being introduced in The Next Generation, the Borg appeared in Enterprise in the episode Regeneration as a prequel to the film Star Trek: First Contact. Exactly how the circumstances were going to come about hasn't been explained, but it was intended that there would be a Borg Queen origin story in season five of Enterprise. It was intended to see if Alice Kringe would return to the role, as a scientist researching the cybernetic aliens that were found in the artic in the earlier Enterprise episode. The story itself was pitched by writers Judith and Garfield Reeves-Stevens, although of course the Borg Queen origin went off in a completely different direction in the novels following the end of the series.