Personally I kinda ship Riku with Xion.
BUT ...
No, I don't think they should give him a love interest unless it is something that actually develops over time over future installments. Sora and Kairi's inevitable relationship has been hinted at over the course of several games now, they just haven't "officially" done anything with it.
Riku hasn't had that sort of direct romantic development with anyone and I would HATE for them to, I dunno...arbitrarily decide he is a pair with someone and say yo, we are a couple now ._.
I think Sora being together with Kairi, with Riku being there as well isn't really going to change their relationship dynamic as a trio too much honestly, because Sora and Kairi already definitely liked each other anyway before, it just finally was put out in the open. Why can't the three of them hang out and be a trio in a game or two without throwing some random other person at Riku in order to make it a double date?
Now, if say, after KH3 and everyone is "saved" and he starts kind of hanging out with someone and THEN we start getting some hints here and there before anything actually happens then sure, I could accept that.
But don't just revive some character (or make one up) only to shove em at Riku and be like "here's a consolation prize. We know you two haven't had ANY chance to actually develop feelings for each other but hey...both of you are single, so...great, now we can have couple power hour!"
No way man. I've been waiting WAY too long for my team of Sora Riku and Kairi to go kick butt together for some rando to come tagging along.