If I could add a NEW theme to the park, I'd want to make Action Zone a graveyard of sorts. Have the rides there that aren't spooky themed be themed around nostalgic old rides like screamin demon, Firehawk, etc. the graveyard of the dead rides. would make sense with the nostalgia in the area with SOB. 

 

aside from that, I think a new area where the theater used to be would be cool. maybe add a dark coaster, add some flatter rides and then some kind of mysterious swamp coaster or something, make it similar to rivertown but not quite. could be the 'abandoned/forgotten' side of rivertown

A creepy woods.

Make it the new extension of Oktoberfest with a Black Forest type setting. The current "mini-Oktoberfest" would stay and the path between there and Delirium would be the primary entrance with a spooky sign.

Oktoberfest fest expansion. 

Making Action Zone into a creepy themed area is a good idea, heck they already have Banshee & The Bat. I'd say rename/retheme Delirum to something more Halloweenish, maybe color scheme it black & orange? Drop Tower would be fine, but maybe play some creepy music in the que like along with mist in the area. Also get retheming if Congo Falls and make it into something similar like a spooky water dark ride perhaps build around it (Disaster Transport Cedar Point). Invertigo, honestly retheme it or just get rid of it, it only runs not even half the time anyways.