Frozen Throne was not a sequel, but an expansion pack. And compared to Brood War, it added a ton of new content. WCIII and FT have a lot of different ways to play between them.
Sequels follow the doctrine of economics. When something does well, why risk making something new? And sequel isn't the best word, but rather formula. Developers like to stick to safe ideas that sell. Only developers/publishers with a good name dare to think out of the box. Nintendo, Rockstar, and similar groups can sell a game just by putting their logo on the box, and thus they can get away with experimentation.
I have seen nothing but ho-hum reviews for the .hack series...reviews that get lower with each of the releases (which is now over with). Not sure about Xenosaga.
That said, Spinter Cell: Pandora Tomorrow had a very refined, but similar single player campaign. The multiplayer, however, is one of the most original and entertaining games I have ever seen. And it was made by Ubisoft, a French company that will no doubt have UK versions already on the shelves. If you want to give it a shot, demos for both multiplayer and single-player are online.