I voted for Moss x Heather and Bigwig x Spartina. Because I'd have so much loved to know more about their relationship. Especially Moss, since we know so less about him, in my opinion. Always a fair guy, but always stayed in the background of the story. And suddenly: Poof Efrafa's new leader. x3"
As for Gluck, haha, I'm so in on that. I guess the producers only implemented her to have an aunt-like character acting funny and childishly, as the main audience still were kids (just like me back then, wow... I'm getting old). But I also never really got to like her, especially since she made Kehaar left Watership Down permanently. (At least until the end of the show)
Overall, tho, romantic also hasn't been such deep topic on the show, similar to the book(s).