Warren Ellis initial script was for 90 minute animated movie, and it didn't even get into the castle. It was supposed to be a trilogy, all about Dracula's Curse. That could have changed but I have seen no indication that season 2 will be about another game, though I would be cool with that.
Dracula's Curse has one of the better stories in the series (with a pretty clear arc and canonical side characters to bounce off of) so hopefully they can at least get this one right since most others would be more of a challenge. Though there are some obvious exceptions like Bloodlines, Portrait of Ruin and Rondo of Blood.
I get a definite Vampire Hunter D mixed with a little Hellsing Ultimate feel from that trailer, and as a fan of both, this could be great. Just as long as the characters are portrayed well and the action and pacing are good.
As for Grant not being in it - eh, as a hardcore fan I'd miss him a bit but he is the least interesting of the original characters in my opinion. And especially without his potential ties to the Dănești family or Ferenc Nádasdy. I could see someone wanting to make a mini-series thinking the whole Pirate angle could be too distracting/tonally different.