Robb13's Big Adventure looks nice but there are a few issues.

Suggestions for improvement:
1) There is a so called "grace period" in a lot of platformer games which allows the player to jump a short moment after leaving a platform.
This will make the game more "forgiving" and walking and jumping feels more fun if you do not have to be perfect whenever a gap appears.
2) Some coins are not collectible without falling into the abyss. Does not feel that great. I know because I collected them all.
3) The skeletons are not a threat. I think they are supposed be an enemy but they hit air and you are not punished for touching them.
They feel like funny little critters.
4) Is there a way to hit the boss without getting hurt? Your game does not have any kind of tutorial and the move set is not clearly communicated to the player.
5) Revise the current levels than add the same amount trying maybe some different challenges.

Feels a lot like similiar mobile games.

How about setting your game apart from the rest?
You could put caves into the level or maybe add a platform which means there are two routes to take (maybe one is more uneven but shorter).

I feel like I have no indication of how far I have gotten. Does not feel fun to me if I do not know how much progress I have made. Especially when I am returned to square one every few seconds (no savepoints).

So changing the look of the environment and rethinking level design could help a lot. Maybe different vehicles.