I would be sad to see the art style change to any type of 3d, especially anything that is pixelated and looks like a children's video game. There are plenty of 3d pixel games to play without turning Flowergame into one. 
I want to say that I am very, very proud of you for your intentions to hire a human artist. Not just because I am an artist that lost my job to AI "art" but because I love that you care about how the game looks and feels. You have my highest respect.
If Flowergame hired artists I would like to get hired as an artist on Flowergame. I could be artist or writer too.
Very late to be responding, but I agree with most of the comments that I prefer the existing art style of the game. There are currently very few pixel art adoptable games left running, and I can't think of any at all that use only pixel art for all of their graphics. That is to say, the game is unique and a throwback to the old internet that can't be found anywhere else.
I also don't think that the fact the art looks old is the reason for declining new users, nor is the fact that it's a slower, less exciting game. There are new games that fit those characteristics, i.e. Stardew Valley, that are extremely popular. The issue is that the way people use the internet has changed.
I can't remember exactly how I found this game, but I'd imagine I saw plants in someone's signature on Chicken Smoothie and clicked on them. And I only played Chicken Smoothie because I saw someone's pets on their blog. And I was only looking at the blog because I was reading about Animal Jam, a flash game I learned about from a magazine. Everything in this chain of events would never happen today because young people don't read magazines, Flash was removed from browsers, no one has a blog anymore, and no one uses forums anymore. People get the majority of their game recommendations from social media and the most common device they use is a smartphone.
Therefore, I think the best way to get new users is to get the word out through social media and make sure the website is mobile friendly. Secondary to that would be adding a few more things to do (i.e. more plants, more minigames to get furniture or maybe special plants?). It would also be nice if after you've reached a certain number of plants grown, the amount of seeds/seedlings you can have at one time increases, because having only 4 seeds at once starts to feel a bit limiting after a while.
P.S. I actually have an idea for a cheap way to get the website advertised in front of 100k+ people, so long as the website is mobile friendly
Dragcave (which is where I heard about this game) is mobile-friendly and uses fixed width, as far a I know.
Yeah I think looking into making it mobile friendly is something I should look into. Unfortunately because of the art we currently use we are pretty much set on a fixed width (at least for now).
Is the art you're referring to the headers and footers? If so, I could probably find a way to photoshop it smaller if you'd like, so you have a mobile size and a desktop size?
