After some thinking i have a long rambling opinion to add.
So having ridden all three variations of the coaster, the best one by far was X in my opinion.
X: No Way Out in its final years suffered a lot of the same problems as TWD, and i guess for some reasons listed above. Like their being bigger, better, and loopier rides in the park which made X No Way Out seem like a glorified family coaster (which it was)..
What made X good was that it embraced its identity, as a family ride. It had a very low height restriction, and it made it a accessible and pivotal family ride for a park that really lacked family friendly rides. This is why it attracted more people and grabbed more attention. I think it also was a crucial 'post lunch' or just something to ride for a laugh. Hence the popularity being a ton more than TWD will ever see outside of its debut year. I also believe that the more 'modern' style than an actual theme was refreshing, the ride was not aiming for world class theming. I feel a lot of the try hard horror themes are really getting boring now, and the more modern and loose 'X' theme was refreshing.
I do not agree with the reasoning behind them changing X, in my opinion i believe it was to bring something new to the park without really spending much. So, bam, slap some Zombies onto our Vekoma family coaster and the public will think we have a big scary new rollercoaster.. But obviously, after 2018, the public saw this ride for what it is.. And its popularity dwindled.
Anyway going forward, i think a retheme or it being SBNO (probably SBNO) is on the cards. I personally think the building probably isn't really big or suitable enough for another coaster, i presume X has a really tight footprint. It would also be tricky to replace it, as Black Mirror is literally uses a piece of the building for itself. A dark ride is completely plausible though, but i wonder if Merlin would invest in another dark ride here after Ghost Train.
I'm not sure if reverting to the original theme would help much. It was, after all, rooted in a very outdated public perception of what happened inside a computer. "Cyberspace stuff" might have been a common way to view it in the 1990s, but nowadays it wouldn't resonnate at all. If anything, X needs something new if The Walking Dead fails.
This is true for X:NWO. But for X, it really did not have a theme. It cut basically all ties to XNWO. It was just a coaster with a bunch of fun lights and blaring music.
I think X modernized X:NWO enough. Even if the theme wasn't even a theme anymore.
When it comes to new themes, i do think something family friendly would be optimal. I feel any successor should ultimately embrace its identity as a vekoma family coaster. It really just isn't a good thrill coaster, XNWO and TWD proved this time and time after.