I really wouldn't mind if they went this way in the actual show.

Spike just gets over Rarity, as he figures he has no chance. Then, because of his time spent always helping Rarity, he's also spent time with Sweetie Belle, so he starts falling for her.

I wouldn't expect them to turn it into a fullblown romance, but it'd be interesting to see what they'd do. Just keep Mcarthy the fuck away form it.