color scemes https://gyazo.com/b7990163f1d59e41ac9b4a722a718d92
try to stick to the same color sceme, as in dont make a midieval well, and pink house.
poly density https://gyazo.com/8df5c76de1d9a36cbbc42e128165d452
try to stick to the same poly density. if you are adding every single plank and rock on a house, then dont use alpha map on trees for instance, keep it the same.
model style https://gyazo.com/f3f98d737fbc9c17e4862fa9a2eea3f3
the left one is actually simular to how world of warcraft does it. another good exsample is wells, and houses, they also have they style some times.
lets say you have a mech, with panels, semes, vents, gears, etc. very high detail, and you have a sci fi enviorment, then add the same amount of details there aswell.
and last, keep the astetic the same. for instance, minecraft uses blocks, there is low poly witch has its own style, anime is its own style, photorealism has its own style, keep it the same.
and most important, know what you want to make before you make it. look for REFERANCE IMAGES!