Now that this rumour has come to light, I've done a little digging on Google Earth at the site they have to play with.
In terms of the valley, I've concocted a rough path where such a coaster could go, based upon the description provided in the video:
If an out-and-back coaster followed that approximate path, Google Earth says that it would be approximately
504 metres long at its longest straight, and that excludes the curve left heading towards Blade.
For some idea, Apollo's Chariot at Busch Gardens Williamsburg, an existing B&M hyper, is
577 metres long, but also doesn't turn like this coaster could, so I certainly think a hyper would be doable if it dropped into the ravine. Or if it were a mini hyper that hugged the ground a little more. I know it wouldn't exactly be a typical hyper, but Nemesis was a unique B&M compared to others of its model, and look how that turned out in terms of reception, so who's to say that lightning couldn't strike twice if Towers had a play around with the B&M hyper hardware and that valley?
On a side note, I almost reckon they could create a sub-area for this coaster alone as opposed to having to integrate it into Dark Forest. Having had a look at Google Earth, the Darkest Depths area is surprisingly substantial; between the Dark Forest food outlets and the public pathway, there's an area of nearly an acre! Take a look:
And to be honest, there's probably a greater area to play with than I've highlighted back there; I do see some trees that I didn't quite catch! As long as they didn't infringe upon the wall of the ruins to the left, I don't see why they couldn't clear some additional land back there and get to over an acre!
Why would you build a massive B&M at either at great expense when you can build a smaller and cheaper Mack launch that'd give you airtime galore on a far smaller footprint?
In terms of why Thorpe would go for a hyper; I'd wager that marketability would be a fairly strong selling point, personally. "The UK's tallest and fastest coaster" is something I can imagine having a fair amount of pizazz behind it marketing-wise, especially with Merlin in charge!
Also, based on my experiences, I'd personally wager that a hyper would provide more airtime than a Mack launch, as while I'd love a Mack launch, I don't think airtime is really the main focus of them, whereas it's usually quite a principal element of hypers. Don't get me wrong, Icon has a fair bit of airtime from my experience, and pretty strong airtime at that, but Mako was on another level entirely; from memory, there was airtime for days on that thing, and it also had a lot of sustained airtime, which I feel that the UK lacks even more sorely than airtime in general. A lot of the airtime the UK has is those quite abrupt pops, that only last for a second or so. A B&M hyper would offer sustained air potentially lasting up to 5 seconds or more per moment, and lots of those moments at that; I'd argue that there's nothing currently in the UK that offers that kind of experience.
In terms of Towers; I'll admit that I feel there are more likely options (a multi-launch like you suggested, for one), as well as more likely sites, but I'm not ruling a cross valley coaster out. I certainly reckon it would be doable if they had the vision and creativity!
For what it's worth, the video also mentioned the deal including a Nemesis retrack... which was already rumoured as is.
