Graphene probably sounds better than BitShares but I don't think rebranding is a good idea for the following reasons:
1. Confusion as what refers to what. (the blockchain, the corresponding core asset and the toolkit) It doesn't make sense to name the toolkit and the chain the same but the asset has a totally unrelated name.
2. SEO. And I think this is a quite important one, at least for the chain. For the toolkit it doesn't matter that much.

We are starting to have each weeks more articles out there speaking about Bitshares. Changing the would be very confusing for people and destroy big part of the communication done so far.
There is also people working being the scenes on BitShares 2.0 projects related. Buying names like
www.bitshares-something.xxx, working on contents where the name BitShares is relevant, ...
As an example, I'm revamping our actual Bitshares French Connexion web site. We would have to change all the media, translation, tutorials videos (xwork in progress), ...
I'm not even speaking about our name and website address.
I'm not the only one in that situation, think about the blog in spanish, bts.hk (the huge chinese website), btswolf, Bitshares TV, Bitshares 101 book, ... the list is very long.
It would add a lot of work at the community only to adjust to a new name. That amount of time would bring much more if used to promote and evolve BitShares.
The renaming thing of IDentabit has been fun and kind of justified but let's just stop there.
BitShares is a cool name, there is no real need to change it.
Let's focus on more important matters.