Well, as we have seen in the never ending romance between Twilight Sparkle and that blue haired human self insert, FiM is not very good and handling romantic subplots. And the ones it does do well, are ones that are either handled in a comedic fashion (ala hearts and hooves day and simple ways) or with more "adult" characters who have already found their place in the universe (ala canterlot wedding). The main issue with having a member of the mane 6 be in a romance is that in order to make it so they have chemistry the character/romantic interest would have to be around often, and partake in the daily goings as a pair. Otherwise the pairing comes off as forced and rather fake, like that of brad the waifu stealer and Twilight the human fetishist. So in order to really achieve this balance, the love interest character would have to be around fairly often, and that would throw off the balance the mane 6 have, where all characters play off one another's strengths and weaknesses. And that is why I don't see a legitimate romance happening right now, the character would either be a total spare tire, and just carried along and forced in only to prove they can do things when the plot says they can, or they would only appear once or twice and be the most empty cartoon romance ever.