I honestly wouldn't care that much about technical milestones in a dark-ride. Maus au chocolat is a great example of this, it's a very well done execution of techniques that were available for a long time (even though the projectors could use a higher resolution upgrade). I think the ETF trackless system or even another suspended darkride would work very well as a Geister Rikscha replacement as long as the overall experience can math the Phantasialand-standard.
Me personally, I'd love to see a lovechild of Droomvlucht and Vliegende Hollander, themed towards an Expedition through the Himalaya in quest of an Tibetan monastery or something. With actual snow hitting your face, thunderstorms happening around you and crevasses cracking beneath your feet.
But I also agree with the assessment that the "old" half of Mexico requires an overhaul, and considering how Geister Rikscha is incorporated within Colorado's station building, I could see this being a 3-4 year general overhaul project, which would explain Phantasialands reasoning behind closing Geister Rikscha this year even though their newest ride will probably not make it in time to make up for the loss.
I think it would make the most sense to start with the Colorado/Rikscha building due to a couple of things. First of all, I believe that Colorado is here to stay for a long time, at least the next 15 years. But as it stands, it's not well integrated into the Mexican area - because it didn't use to be part of it - and park officials have stated before that they are not happy with the location of Colorado's entrance. It's also located towards the back of the park, so it's blocking a potential construction site exit. And to rip out Geister Rikscha, they need back access to the building as well, since I believe ripping out the thing will be much less troublesome than adding in the replacement ride. Meaning: Mexico needs to be finished before they can replace Geister Rikscha, since it's possible that the construction site will render most of the chinese area unaccessible for guests, and I don't believe Phantasialand wants two out of it's six areas to be under major construction.
So what would they do with Colorado in the meantime? Honestly, not much. Give the Rockwork a refreshment like Deep in Africa has received it, give it new comfier trains and redesign the station building to match the Mexican theme. Including a new name, because Colorado... well, we all know the geography-issues. I believe the heavywork on the station building can be pulled off during the off-season in 2020, so that Colorado wouldn't even have to close for a single day. And of course, move the entrance towards the front. Park officials have stated before that they are unhappy with Colorado's entrance location, yet acknowledge how there isn't much they can do.
What will have to close though is the Silverado-theatre. It's the most outdated out of all the theatres in the park and also the least effcient in terms of space consumption. It leaves some unusable space towards the back end of the park while in itself being overly large, so I believe this one will be scrapped entire and replaced by an all new building in a location as far back as possible to eliminate the narrow populated pathway in front of it - that would give Phantasialand the opportunity to create a nice open plaza in front of Colorado Adventure. I also believe that a Silverado-replacement will house a replacement-location for Fantissima. Yes, the Temple building has received some upgrades recently, but Fantissima is expensive, and the overall state of the building has become close to unpresentable. I don't think it means that much. The days of that building are numbered, but it won't go until Fantissima can go somewhere else. The Silverado-theatre is accessible from outside the park, it has a large footprint and pretty much needs a ground-up replacement. It's the perfect (and really the only) fit. It also moves Fantissima very close towards the two main hotels, which just makes a lot of sense.
This could probably be finished towards the beginning of the 2021-season - Silverado being an all-season construction site, same for the Geister Rikscha half of the building, but all the areas that can't be worked on while Colorado operates would only be off-season construction.
By that time, Geister Rikscha will be long gone and the above ground part of the new station building might even be finished already. That leaves the underground space to receive it's replacement dark ride, which - unfortunately - will probably require the entirety of China to be closed since they'd have to rip the plaza out. However, the park seems to prepare for that as River Quest will most likely have received it's new entrance in Klugheim by then, and Hotel Ling Bao is far enough off to the side that it won't be affected by the heavy construction too much. Also, since the Colorado entrance has been relocated, that pathway could serve the hotel as a park entrance towards the renovated part of Mexico. It's not going to be pretty, but I don't feel like there's much better options unfortunately. Who knows, they might even close Ling Bao for one season entirely (even though I doubt it).
Since China would be blocked off entirely, that would also give Phantasialand opportunity to work on the other parts of China that are not directly affected by Geister Rikscha. For instance, the food stands located towards the China-Mexico pathway within the last remaining building of the Silver mine could receive a complete overhaul. The vacant building could either be scrapped entire to allow for a much wider pathway since it's always densely crowded, it could be used for another higher-tier restaurant or it could even be turned into a small indoor-ride. I honestly don't know, but I feel like something will be done with that during the next few years since at the moment it's blocking precious space.
Feng Ju Palace might also receive an overhaul with improved theming and pre-show, but I think the ride is there to stay. It compliments China as a family-oriented area of the park, even though it's not revolutionary or anything.
So if all goes well and my predictions are somewhat correct, we could see two theme areas having received an extensive refreshing, including the opening of a world-class dark ride by 2023.