On there being more writing associated with Brood War intended, I highly doubt it. There's a lot of evidence from Wings for example that they saw themselves written into a wall back in Brood War and made several necessary omissions to make SC2 flow better. Explanations followed for these omissions, such as QoB going back to a more SC:O style, Fenix nor UED ever being mentioned, the new Overmind not being mentioned, the Dominion re-arising completely to it's pre-BW state. In short, they didn't like where the story had gone in Brood War and decided to change it.
If you think about this, it could be more evidence that it was a scrapped storyline of BW. Fenix and the UED not being mentioned (not directly anyway), as well as the recovery-ish of the dominion.
I could easily see raynor being in a similar position after the Overmind is slain. He'd obviously want to deal with arcturus, who pretty much does not get mentioned for 2 whole campaigns (one in SC1 and one in BW) and about half the zerg campaign (SC1)