Here's a couple of things that I personally think adds a lot to your game and to me shows that you put a lot of effort and care into your product. A lot of these additions are things that most players probably won't really think of consciously but nonetheless adds a lot to your game. None of these things will take very long to add and I really wish more makers would pay attention to these.
This is the biggest point of this list for me. Adding background tiles is such an easy way to make your game look significantly better. It's an easy thing to overlook and not even think about when you are in the graphics stage of gamemaking. Even though the player probably won't think about the tiles consciously, if you do a comparison of before and after adding the background tiles, you'll definitely notice how much of a difference it makes.
Here's a side by side comparison of the first screen in the game I'm working on, with and without background tiles.
When I say have a story, I don't mean you should have a full fledged story with developed characters, a plot twist or anything like that. Even the most basic story for goes a long way imo. Give the player a sense of an objective, rather than just standard platforming with some gimmicks and areas. I wanna run the Marathon is a great example of this, the story is nothing big and it's just a silly little side thing. But for me, it added quite a lot to it.
There are many ways you can execute a story, it doesn't have to be told to the player with a textbox such as "x person is bad, you are the hero go save the world". It can be told through the environment instead. A good example of this is Alexbrogan69's game, I wanna see my Friend. In this game, you go on a basic adventure to see your friend. Suddenly you fall in a big hole and once you're at the bottom, you'll climb back up doing all the screens you saw on the way down. At its core, it's just a basic needle game, but the added "oh I have to get out of this big hole" adds a lot to it and gives the player a sense of objective.
This point is mostly just for gimmick style and adventure games and it basically comes down to not overusing a gimmick, stage, area etc for too long. I wanna find my Destiny is a great game and I'm sad that I can't enjoy it due to the ridiculously long stages. Unless you have a billion different ideas for each and every stage, the screens are going to blend together and it won't be very memorable.
This point also goes unnecessary jumps & spikes in needle. In a lot of needle games, often from new gamemakers, there will be an insane amount of jumps and obstacles that feel like they were only placed there to make things more difficult, not really respecting the fun value of said obstacle. Know when to stop, not every nook and cranny needs to be filled with spikes. It's ok to give the player a break and have a small area that feels a little empty. The easiest way to avoid the "not knowing when to stop" problem is to severely limit the amount of minispikes that you use.
On the topic of find my destiny, another big gripe I had with that game was the complete disconnection between the stages. This one also ties into the story point, there is zero sense of objective when you are just teleported between stages without any reasoning or explanation of why you're suddenly in a completely different environment from the previous stage. For me this results in stages that feel arbitrary and a world that doesn't feel alive.
Once again this can be applied to needle games, although is nowhere near as important. I've always prefered when needle games make you leave the screen border to go to the next, rather than having a portal on every screen. This point is very minor though so you shouldn't worry about it too much.
Obviously bosses are quite hard to create and takes a lot of time, so why is it on the list? Well for this I don't necessarily mean the process of creating a miniboss, but rather the idea of it. Having a small break inbetween the bigger bosses definitely helps keep the flow of the game, especially in a game like find my destiny where the stages are extremely long. Having a bunch of fairly easy minibosses and fewer difficult big bosses will make the bigger bosses feel more impactful aswell. I really feel like games need more 30-60 second bosses that aren't too difficult and makes the player think "hey, that was neat!"
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Please bare in mind that these are just my personal gripes and pet peeves I have with fangames. None of these points are objectively better than their counterpart, they're just requests I have. If you don't agree with me at all, don't let me stop you from creating whatever you'd like to create.
Happy gamemaking!