If there is a series of flats to replace the Retrosquad, the park's main concern should be throughput and reliability. So they should go for flat rides that are fairly established in the industry.
If they need some USP I'm sure the park is creative enough to design a theme that will sell it, whilst still fitting into the area it occupies. I'm not one for "it should exist within the continuity of..." and I'm more than happy to have an attraction that serves as a unique extension of the area, similar to how Burial Grounds serves as an anarchic successor to the main Wicker Man story, or how Sub Terra serves as an extension to Nemesis.
Personally, it would be great if new flats also complimented each other on the lineup. At the very least the park needs one more pendulum-based ride and one more round ride. With at least one being more overt thrill and another more family thrill. Something airtime-based like Mixtape was would be a crowd-pleaser for sure also.