Coaster Corner IS an option, but it has to be designed to ensure it is "sound proofed" well enough.
The Black Hole spot too is possible, but again there are noise issues, plus the site is actually pretty small.
The Flume area is ripe. The trees aren't protected (AFAIK) and the wooded area will help to kill noise from the ride. The Flume area has also been tagged as a potential site for a new water ride - that could be altered to coaster.
Gloomy Wood I think sits on some protected areas. The feeder lakes
may be historically protected, I don't know to be honest though, but again, there's not actually that much room.
The entrance to Forbidden Valley (the old Dynamo site) could be used for a coaster potentially, running around the top end of Nemesis and the carp park, but height is seriously limited there.
There was talk of moving Spinball to the edge of air on the lower car park. This is another potential site for a coaster, perhaps dipping into the edge of the top end of the valley.
They're really the top sites, and I'd be leaning towards the air/carp park and Flume sites personally.
As for 13, the only fault with the ride was how it was marketed. It is a well themed, low thrill family mine train ride. It's a perfect replacement for Corkscrew and it was installed in a protected woodland without compromising the building regulations enforced on Alton. Only a small ride like 13 could do it. They added the drop to try and give it "thrill" element, and it's not worked 100%.
It's certainly a flawed ride, but there is nothing that could go in it's place, except maybe
another small, low thrill family coaster.
It is what it is and not what it was advertised as. Don't get the two confused or you'll be saying that you've been on the tallest and fastest coaster in Europe without leaving Lancashire