ideas (and % likelihood, in my opinion)
1. have things only pregnant women can do (35%)
maybe, while they're taking care of babies, they can network with other pregnant mothers, and this would increase their parenting. would also make it possible for women to be elder a lot sooner than otherwise is feasible.

2. upgradable buildings (75%)
maybe the huts get stronger
maybe they become two stories
maybe a man who is an elder, and father of at least ten kids, can build their kids a tree-swing

3. activities that will allow the adults to build in more than one field at once (50%)
the tree-swing (#2 above) could be an increase in parenting AND builder

4. upgradable accounts (70%)
make the game completely awesome in and of itself, but allow importing from a completed VV1 or VV2 game (defining completed as having finished all puzzles, reached a certain population, or whatever) and allow those to have access to "five hidden levels" or faster tech point gains or maybe one free tech level at any point of your choosing)

5. freewill gauge (20%)
i'd like a visual way to determine the likelihood a villager will actually act out the profession i've chosen. if I've assigned the villager to be a farmer, i'd like to know how enthusiastically they'll engage, or whether I'll have to babysit until they become adept/master

6. flow toggle switch (72%)
i'd like if there were a way to see how close to improving/leveling up a villager is. As it stands, the only tech we can track with any reliability is researching
say a certain combination of flowers in a mortar and pestle by a master scientist would reveal how many "this villager is improving at XXXX"s are needed before the villager in question improves
or maybe a toggle on the options screen to show white numbers in the middle of the sliding bars to see numerically their advancement

7. a seventh tech level (15%)
i'd like, as a complete surprise, that once you finish all 3 levels of each tech, that you have one more 3-level tech to finish, that has game-enhancing qualities that are separate from the other six, but complement them somehow

8. elder necklaces / mentors (80%)
instead of elder totems, how about elder necklaces? It makes sense that an elder would want to pass on their knowledge to someone else, and it seems shame to lose all that built of knowledge - what if that knowledge could be "stored" in a necklace, and they could give it to 1 child. that child couldn't use the knowledge until they were an adult, but would gain some benefit from the necklace, like..
A - automatic mastery once the adult makes adept
B - learn that skill twice or three times as fast
C - doesn't gain faster themselves, but anyone learning the skill with them gains faster because he/she's a natural leader

9. plot resolution (90%)
i don't know if its possible, but I'd like a resolution to the plot. VV1 and VV2 have a kind of single story of which they're a part, and I'd like if the resolution to the story were contained in this final installment to the franchise.

10. food for tech (10%)
i'd like, once you reach third level food or science, if you could exchange food for tech points or vice versa

11. how about recipes that (50%)
improve speed (not much, maybe non-cumulative 10% increase) in a given tech
increase the likelihood of having a boy or girl in childbirth
tweak (randomly) the preferences (likes/dislikes) of the person who eats it
makes them villagers immortal as long as a certain puzzle isn't solved
makes a man pregnant
causes a random event to occur on the island
makes all the elders 30 years older and no longer elders
randomly switch skills (once a master scientist, now they're a master parent!)
makes an adult a child (temporarily or permanently)

12. Gilligan's Island references (2%)
personally I'd be entertained if some sort of reference were made to GI during this last installment - maybe a random event that looks suspiciously like a coconut radio or maybe a set of outfits that look like what the GI residents wore

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"The most pathetic person in the world is someone who has sight, but has no vision." - Helen Keller