To answer the thread title's question, I'm going to say the original Antique Cars. Paramount was bad at putting things where they didn't fit, like building IJST using less than half of that land instead of Action Zone where it would actually fit the theme of the area. Action Zone itself was meant to be a backlot, but instead they bulldozed a large area, an attraction the park itself was designed around, and just thunked it down there while leaving the rest of the property a bunch of flat grass.
Regarding Diamondback, I'm sure it would be done differently now in a more area-fitting way, but the early Cedar Fair years focused more on catching up on things Paramount had lagged on, especially the spree of building B&M hypers. Maybe if Paramount hadn't done IJST how they did, Diamondback's station area could have taken the place of the closed Rivertown Antique Cars area and crossed the pathway using the lift hill instead of the drop, without removing the lake. The return run could have been a tunnel under the midway to get back to that side of the path for the finale element and brakes.
Obviously there's no way to bring back Swan Lake as it was, but there's a lot of open grass on both sides of the pathway there, behind Showplace and also across from Diamondback that's sat vacant for a very long time now. A new lake as an entry to Rivertown could use both of those areas, with the pathway bridging over it. Diamondback's repaint is a welcome step in making it blend in more, but things like landscaping around the pool, upgrading and theming the queue, replanting the excessive number of removed trees behind the former Crypt building to better obscure the back half of the ride, there's plenty of things that can be done just with theme and landscape work to fit the area better.
(There is still a box of broken flip phones that forms the "theming" for the queue. That got old ten years ago, guys.)