I hope that when Subu sadly dies they will not just replace him with a non breeding pair in the same enclosure
You are right, they could/should do a larger/more roomy enclosure in the 'edge of Africa' area before contemplating more Lions.
I have the ideal species to replace them in the 'Lion Rock' enclosure. It would- with some landscaping- make an excellent enclosure for the Gelada Baboons- imagine 30 or more of them running about in there against the concrete 'cliff' backdrops and playing in the sanded areas, . 100X better display than where they are now.