Wow I didn't expect so many and useful replies. Thank you guys.
I did the fresnel thing and it really brings the characters out from the background. I made it only on the edge pixels, and as the game is 2x scaled, it is kind of sub-pixely. I like it though, because the player sprite is so small, the full 2x size bright pixels would brighten half the character.
For the lights, they are my number one enemy. I would really like to make them smoother and bigger, but then the player could place more of them next to each-other and it would make a really bad bright effect. That could be dealt with by using separate Grabpass for each light, but then the player could place many lights on the screen and destroy the performance. The lights have shader which brightens colors only under some threshold, which prevents them from blinding people during the day. There should be a better way to do this, but I'd have to redo everything concerning lights and ambient light.
The wooden floor tiling didn't even occur to me, will redo. The wall top sides really look better color-separated from the rest, shame I will have to use more colors.
EDIT: Oh, forgot about the visible area concerns.
So, I agree it looks and feels better when smaller area is shown, but the problem is how to implement it? Now the sprites are fixed size - 2x the size of the screen pixels. I could zoom in more depending on screen size, but I'm afraid it would look really bad to have 5*zoomed pixels on fullHD screen. My animations are not good enough to look nice close-up. Also there is spell-fighting and the spells can fly quite fast, so reducing player's view could have negative effects. Guess I'll have to experiment.