As much as i am a fan of romance, i am going to agree with
Nightengale: While the potential is definitely there, i don't expect the directors to venture much further than the (very close)friendship area.
I would say there is enough material to make both Nanoha x Yuuno and Nanoha x Fate (even though my preference lies with the former) easy to pull off, and no one would be going "wtf, where did that came from" <- i think we can all agree to this.
However, i don't see that happening. They are, probably, going to tease us with hints here and there, but nothing conclusive in the end. (The very end of the series is the soonest i would expect any romance revelations on this front).
Erio and Caro seem like the most obvious one to me, and one that actually has a very high chance of happening (as far as their age allows it to, assuming their age is being taken in to consideration). They sure are not hesitant about dropping early hints about this one, and if we are going to see any "couples" at all in Nanoha, i think it is a safe bet to say that Caro x Erio is the most likely one.
Something like Teana x Subaru and others don't even seem significant enough (and too random in my books) to even mention
