Wind and fog
During and after heavy rain have the water level from the river rise
Snow actually having some depth after a snowstorm
More trees
Give villagers more of a unique personality
Add vegetables as a gardening option
Add cooking
More public work projects

Those are all great ideas!

What roles would those be?
In addition, Pokemon is based on combat, but studies have shown the playerbase is mostly female, including numbers as high as 63% by some studies.

Decorating a house, making a new outfit. I know that sounds really sexist. Maybe I should have said that a lot of the gameplay in AC is geared towards casuals. But then again, I'd probably get flak for that one too. I've got over 200 hours in New Leaf, and I'm no casual. Anyway, I just think that doing something other than fighting bad guys, is part of what makes AC such a unique and popular game. 

Haha, don't worry, I am just poking around. I wasn't serious about the combat thing either. I don't think AC needs anything like that.

Anyway, as far as casual gaming goes. Casual games most often have combat of some sort, usually that or some kind of slot machine.  As for casual gamers, I don't think such a thing really exists (or at least it is too poorly defined to be a true thing), it's usually a term that certain gamers use to disparage other gamers habits or preferences - kind of like calling them stupid. Usually coming from people who call themselves 1337 or hardcore gamers, both kind of pretentious terms.

A way to keep your favorite neighbors from moving away cause you're afk for a month or more needs to happen. This is what killed Wild World and New Leaf for me.

Nah, Goofy had Pluto so it's all good :D

I know these would be hard to execute, but I've had two ideas...
The first is an actual city, with tough city animals and mean landlords and ineffective public transit systems...
The other idea is to make a game that's more photo realistic...
Instead of living next to an anthropomorphic talking rhino, live next to an actual talking rhino...

can you like marry an animal in town in any of the Animal Crossing games? or develop more of the personal life stuff, relationships, official BFF-ships, form parties for friend animals birthdays, plan events for the town?

I don't know, seems like there is more stuff that could be done in terms of making the town experience unique for you depending on what you pick and choose. 

That would be cool. If they want a bigger world, perhaps starting with one biome and expanding later on would be a smart idea. The villagers would also be suited for their environment. Camels would go to a desert, penguins in snowy/cold regions, etc.

This is Animal Crossing, not Pokemon :P