By my knowledge (which is limited XD) im not aware of any ovas,or follow up series(i know this for sure)., why you ask,
Well, you could do a story on the two babies, but where would the extra monsters come from? *has an idea about a possible second series.*, i know what would make a good one, but i like to think to myself.
I think Blood+, is perfect, and perfectly ended its series.(one of the best, because if gently wraps everything up, and will leave you make assumptions at the end, but not stupid ones.),
a good closed anime, i dont think there's a need for one, but i would love to see another one, but that would call for a time skip if you wanted saya back in the picture and having to go through regaining memories and all that painful stuff.
the interesting bit about blood+, is what will happen to the babies, and how they react when they cant age that well., speaking of which, does that not mean, they will still be babies even if you skip ahead to saya's re awakening?.,