Making a good overworld with modified newfirst is very hard. For beginners it's easy to make good overworlds with the pure tileset, but that is far overused. Classic is easy to make good overworlds with if you have a modified version of it, otherwise there's not enough tiles in it to make a decent one. Basically for new people, try to stay away from the newfirst tilesets and the gameboy tilesets. Of course you can use custom tilesets, but i'm talking about the main tilesets that people use that are seen more often.
You can try to change tilesets and rip the custom tiles you made, and once you get more experienced at making overworlds, you can switch to newfirst and make a remake of it or something if you like the way newfirst looks.
The main problem is the emptyness. The only real way to get better at making overworlds is to make them. Through experience you gain ideas and develop your own overworld style, whether it's unique or not. I see you have a problem with things touching. Like, the overworld looks like a kind of overworld were trees cant touch houses, mountains and hills can't touch water, etc.
Although that's usually for people who don't like things touching, some people who don't mind, do it subconsciously. Let things touch and it will open a new world. You will be able to fit much more in one screen. To tell you the truth, a simple 11X16 screen is NOT enough room to make elaborate overworlds and dungeons (Especially dungeons if you have two-tile long walls on the sides, unless it's free-form style.) So, to make up for small screens you will need to compact everything together moreso than you would think you need.
Personally I think that is your only true problem. Yes, your overworld is very expansive and looks good from far away, but to tell you the truth, it isn't that great up close. I expected good things looking at the picture, but then once I loaded up the quest, I wasn't impressed. I understand you're new though.
Almost EVERYBODY starts out making bad overworlds. Heck, my first overworld was a barren land with armoses in the original ZC graphics. Then, I made my first quest. It had grass,trees, and ruins and buildings, but it just wasn't good. But now, I consider myself fairly decent at making overworlds. Although i'm not as good as the bigger questmaker's names out there, through experience i've gotten better. I think you need that too. Experience, and also start to build things closer together.
I tried giving you advice that most people don't give. I check these forums day by day and see sooo many overworlds like yours. They always ask for advice, and everybody says the same thing: it's too bland. However, they don't tell HOW to make it better. And if they do, they always say the same thing: put more in it. Well, i'm trying to take a different approach. DFW left a better comment than most I see in this section for people asking help; you are lucky. If you took the time to read this comment then I hope it helps you a lot in the future.
EDIT: Woah! I posted twice on accident. Sorry about that... I only clicked the button once... Weird.