I have a game which is open-world-ish and you can walk around and fight enemies and fight bosses too. But the game seems "too easy" now. You can basically just spam your attack button and every enemy is easy. Even the bosses.
... And your solution is stamina. What are the alternatives?
A classic solution for this is to put a cooldown on attacks.
However, let us go back even further... You can have the character committed to an animation. Dark souls does this too. That could be enough for your situation.
Other solutions include attacks push enemies out of the way, so that spamming attack does not hit them. Also using an invulnerability period (a fraction of a second) to enemies after each attack, so that you will get hit if you try to spam.
Alright, so, this random person comes running at you and start poking you with a stick. What do you do? Block? Disarm? Run away? Hit back? Get staggered and wait until you die? You can make enemies harder by having then react smarter.
These all serve different goals in your design. You must think what kind of play do you want to encourage. Should, for example, the player be calculating every attack? Or should the player be taking more ground after each attack?
You also need to be careful to communicate whatever mechanic you pick.
The problem with stamina is the down time. You do a burst of attacks, and then you have to step back to recover it. Meanwhile, what is there to do? Not much really. It is down time. Not engaging. Boring... Oh, an enemy is attacking! Avoiding it takes stamina too? Darn you stamina!
Taking this to casual players, for some definition of casual, will probably go wrong. If your target audience is people who plays in small chunks of time, then value their time!
The same goes by giving invulnerability phases to enemies, and designing combat based on waiting for the enemy to lower its guard. Value the player's time.
If you are going to use stamina, you need to give the player something to do when they have no stamina. Better yet, give the player something to do about it.