What do I mean by this? Well, I'll just explain how it might go;
0. When you click on an empty Save Game Planet, the circular buttons for the different stages are split into semicircles(imagine it like the
emote without the text). The green halves are buttons for the vanilla Land Route, which plays like normal, but the Blue halves make up the stages for the Water Route, which evolves a sea creature instead of a land creature.1. Cell Stage doesn't need any alterations by itself, though of course finishing it needs a change. I'd imagine once you're done kissing Maa and Paa goodbye(see what I did there?
), you can either choose to go on land, or stay underwater. 2. Aquatic Creature Stage. This one is the most like the vision of the Aquatic Stage was before it was scrapped, only it's an alternative route instead of bridging the Creature and Cell Stages. Of course, you swim around evolving as a sea creature, either hunting or socializing with others. Carnivores eat the meat of other creatures, while herbivores could perhaps eat seaweed, plankton, or algae; omnivores can eat both, of course, and all 3 types are fair game to eat some caviar from their enemy-anemone. On that topic, the Nests could be anemone, and have more fish-like eggs. There could even be a mechanic counterpart to Gliding, having certain parts that let you swim closer to the surface, or just let you swim in all directions to set the feel apart from the Land Route. Or, maybe instead of using wings to Glide upwards, you could use advanced gills or anglers to go downward into deeper waters? Lastly, trees and other flora could be types of seaweed or coral.
3. Aquatic Tribal Stage. Basically the same, though with Fishing and Land-Hunting being essentially swapped(I can imagine some tribesmen swimming up to the shores and baiting land creatures like sirens or kappa
). And maybe instead of the Chief summoning fish from the sky by the Flying Fish ability, there could be land animals taking a dive from the surface to drown? The idea of Flying Fish feels a little more morbid when the roles are reversed like that... 
4. Aquatic Civilization Stage. To get the first thing out of the way, I'm sure the terrain should change so there's more water than land, so that it doesn't feel like squeezing in between continents. But, more interestingly, the vehicles and Spice Geysers. Spice Geysers could appear underwater, and could shoot out bubbles of spice rather than a constant spray. The vehicles could have tanks that let the drivers go both on the seafloor and the surface, maybe even ones that dig caves into the deeper parts of the continental shores. The planes and boats could merge into submarines, of course.
5. Space Stage. Aquatic Empires have water inside their communication rooms, like fish tanks or those creepy Sci-Fi incubation-tube-things. Their Simlish could even have a bubble filter over it. Maybe for both Routes, you can have both land and water colonies that use atmosphere domes for their respective Empires?
6. Galactic Adventures. Playing a Water Mission as a Land Captain gives them an air-tank attached to their spine and a scuba-mask wrapping around their mouths. Likewise, the opposite happens when playing a Land Mission as a Water Captain gives them a water-tank and mask. If my wording there is confusing, think of it like in SpongeBob Squarepants; where sea creatures enter Sandy's tree-dome with fish bowls over their heads, while Sandy uses her suit to go outside of it.
Of course, I'm not to experienced with coding so I can't really confirm if this is possible, but this was just a fun brainstorm I wanted to share more than a stingy request. Even if it's a tall task, I hope you got some inspiration regardless! Of course, replies adding onto this are highly appreciated. Thank you for reading!
