I've had a few thoughts, but I do think the next SW will be aimed at teens/adults, with SWX (SW10) another thrill/family. Reason being, SW5 Air (high thrill), SW6 Th13teen (family thrill), SW7 Smiler (high thrill), SW8 Wicky (family thrill), so if the towers are going by this model (which I think is a sensible one), it makes sense that the next will be a more intense ride with a 1.4m restriction. And with quite a few people saying Icon looks better, they will want something to draw the attention away from Blackpool. Here are my ideas:
1. I've said this before, Intamin Blitz, look at Taron, amazing. Alton could do a similar thing an with the theming skills from Wicker Man, have its unique marketing be 3/4 launches (no1 out station, 2/3 be somewhere along the track and 4 be a backwards one). Theme? I think a steampunk one would be cool.
2. RMC Trex: The coaster itself is a unique image selling point, no T-Rex has been built yet, boom worlds first. They could make the rides theme like the name, with dinosaurs.
3. Intamin Aquatrax, no seriously, these things pack a punch, can launch, can probably invert, theme could tie into the towers and chained oak or something. The park is lacking another water ride and unlike other watercoasters, these use various sprinklers to spray water on the rides which equals less maintenance and cost (which merlin will love) and a water coaster is pretty unique, so boom, marketing right there.
So thats my opinion, as for location I don't really think GalacticAir's car park is a good location, I think the old coaster corner site is a good place, demolish charlie and have an area there. So Here
Katanga Canyon is a difficult one, I could see something there, but the area would have to expand to the area behind RMTs helix: Here
One I don't ever really see happening is behind X-Sector, but its just a thought: Here
Finally Gloomy Wood, remember that application for a Vekoma around the haunted hallow? Maybe they could have a shot at it again: Here