I've gotta disagree with the whole 'family friendly' approach you mention in your post, @Matt N. Mainly because most parks in the UK are already primarily focused on this, especially in the south. Legoland, Chessington, Paultons, hell even Adventure Island are all primarily family focused parks first and foremost. Having a theme park down south whos main focus is purely on thrills is much more likely to benefit them in the long run as it's a good niche for down south.
Hell, even the parks up north for the most part are becoming more and more family focused - Lightwater Valley, Drayton Manor seems to be heading that way too, and even Alton Towers have got an extremely large predominantly family friendly section of the park that seems to only grow larger as the seasons progress, meaning more thrillseekers will be more likely to travel down south to Thorpe to get their adrenaline fix (or to BPB I guess).
But I have to agree on Thorpe needing a good dark ride, just not on the 'make DBGT tamer' thing. A good, high quality, actually scary ghost train would be a perfect fit for Thorpe Park. As Thorpe Park is a thrilling theme park, a thrilling dark ride such as a ghost train is the perfect fit. They would be seriously missing a trick if they just decided to get rid of the ghost train altogether - it just needs improvements such as replacing the windows on the train with television screens, making the movement scenes to and from the underground platform longer and have the actors that currently stand around and brush your knees actually, you know, act. Perhaps like they're infected or something? idk. Bring back the train crash sequence in the tunnel (but make it look much better and increase reliability) and then back on the train for much of the same.
Either that, or just rip out everything inside the building (the exterior can stay, I actually really like the exterior theming) and start again with a trackless darkride ghost train instead.
Thorpe also really do need a good quality water ride now that Loggers is in the Thorpe archives. Due to good ol' British weather, I think it'd benefit from being inside. Maybe build it on the island behind The Swarm, or if you wanted to save that space for a new coaster, demolish Slammer and Black Mirror (and probably even TWD:TR) and that frees up a decent amount of space for either an indoor water coaster or an indoor log flume type beat.
Swap out the majority of Thorpe's flat rides, too. Rush, Zodiac and Samurai are all well past their sell-by dates. Just look in the Zamperla catalog and blindly pick one - I'm sure whatever you choose will still be a fantastic fit for the park.