This park's big enough to accomidate one more major and one semi-major coaster.
Out of interest what do you qualify as a major attraction and a medium attraction? I'm sure they have space for more than 1 medium size attraction. Adventure Island has atleast three (Green Scream, Beelzeebob's and the Logflume). Dreamland is bigger, and I recon you could atleast two medium size attraction besides a major one. It can be deceptive, but you'll be surprised at how you can pack attraction around each other. On the 'I Dream of Dreamland' plan there are the following:
Major Rides:
1)The Scenic
2)Tsunami (Tidal Wave at Thorpe and Hydro at Oakwood are both classified as major rides so at Dreamland it would also surely fit this category)
3)Wind Walker (Perhaps this would be a modern equivalent of the Lady Bird which park visitors generally counted as a major ride)
Medium Rides
1)Loot Fume (A Log Fume can easily be a signiture ride, but if this only has 1 drop I guess Jim would count it as a medium size ride)
2The Flying Fish (Had trouble deciding whether it was a major ride or medium size. If you compare the layout to something like Dragon's Fury then probably medium. But some people counted the Wild Mouse as a major ride and this looks atleast as big).
3)Squiddy Scenic (perhaps a small ride, although it's quite ambitious for a junoir ride, maybe a bit more so than a mini apple).
4)Storm (whether a medium or major ride depends on the qulaity of the pre-show etc. But certainly more than a minor peripheral ride).
5)Silly Surfin' (having ridden one I recon it's definately big enough to qulaify as a medium attraction, although I won't class it in the same league as the Scenic)
6)The River Caves (again could be a major ride, but perhaps not judging from the size of the building)
7)Island on Top of the World (would certainly be a significant landmark).

Sea Serpant (any UK first looping coaster can't be described as small. If Thunderlooper was a medium ride at A|lton Towers, a similar style ride can at Dreamland)
9)Cannonball (although it is hard to designate a ride category without an indication of height)
10)Crow's Nest (if only because a topple tower is fairly marketable)
11)Blackbeard's (although Sally have a reputatation good enough to create a major ride)
Actually the more I think about it the more your estimation seems a little stingy. Then again you might be right, at this stage it is hard to tell. Am I right in thinking the I Dream of Dreamland is roughly to scale?