Characters are Like Plants in a Garden.
People discuss at length the pros and cons of systems and settings and all that jazz and while all these things are somewhat important the thing that most players remember is the characters they play and their specific stories.
Like Gardens where you place the garden, what soil you use, and tools you use are important but all of that falls to the wayside compared to that beautiful Rose Bush you grow. The PCs are these plants. Each is unique in their own little ways. Some have more flowers and are more memorable then others. Some die early on they don't do well with the setting (soil and water) or they may end up over worked and die from doing too much (system).
The soil, sun and water you find in a given garden is the single biggest determination of what plants can and will grow in them. I could try to grow orchids and corpse flowers in Minnesota's short summers. It won't work. It will struggle and die. I could also try to play Superman in a B/X classic dungeon crawler. It won't work. Settings are the most important thing for deciding if a given character will work.
The player deciding what seed to plant is the most important to decide what will grow. If they go "I am going to plant a fighter in this garden" you won't just get a wizard (usually grafting is a thing and in the allegory of a garden that probably is our multiclassing).
Some characters and plants need a little more to get along then just soil and water. That Coneflower is the basic fighter and you can grow them basically anywhere. They are nice
But sometimes you want other stuff though. How about some Wisterias?
Oh wait these need more then just sun and soil. They are perfectly good flowers and there is nothing wrong with them but you need to give them a framework to build off of. This is your system. They need something to build this beautiful character off of. They would wither and die without this. The character left behind and forgotten or struggling along in a setting that just doesn't work for them. But give them a simple wooden frame to grow off of and they bloom into a beautiful thing. This is the player that wants to play a werewolf. He can't do that with some sort of mechanical support either from the get go (a preinstalled frame) or the Gardener/GM puts in some sort of framework for him to do this thing.
You must be careful though. Sometimes the system designed to help prop up the character can become overpowering and you lose sight of that beautiful character in its midsts. The ornate iron gate.
Your flowers just got lost in this. It hides what is truly important to your garden. This is your system that has so many rules and restrictions that everyone just says "oh I played a 5e game" and you lose sight of that beautiful Druid you played with the little red panda looking flame elemental you played. How she worried about her village back home and fought and struggled to provide aid for them.
All in all know that your garden of a campaign can and likely will grow into a beautiful thing with some consideration as to what you are doing there and occasional weeding out of problems. Don't feel to bad if some of the plants don't grow. It happens not every garden fits every plant. Not every character and player fits every game.