I find fault with a few things in the 5E magic system.
#1 No spell has a 100% chance of success.
Period. Some spells have very high chances, some have very low chances. It also depends what or who you're casting the spell on. (IE. The lock gets a saving throw against Knock.)
For example, in 5E the Identify spell just says, "If it is a magic item or some other magic-imbued object, you learn its properties and how to use them, whether it requires attunement to use, and how many charges it has, if any." This implies that you just *know* all it's details, no error.
Uh, no. You have a *chance* to know all it's properties. Sometimes that chance is very good, sometimes very bad. I will tell you how certain you feel about your determination.
On the flip side more than one character could cast Identify on an item and see if their results matched. They could also go research the item or consult an expert and cast it again with a higher chance of success. Etc.
#2 Spells can be cast jointly.
If two casters cast the same spell together they have a higher chance of success, damage output, correct divination results, etc. Spells can also do things they would not normally be able to do. (For example two casters on different planes jointly cast Teleport. The traveler has never even heard of the destination, but due to it being a joint cast with one caster at the target location the teleport succeeds without error. Situations like this I will adjudicate individually when they come up, but let's just say I reward good teamwork.)
THIS MUST BE MENTIONED WHILE CASTING! You cannot say "we're working together" after the spells are cast. Also the casters need not be near each other, they just have to have some form of communication. Be it visual, auditory, or even something like a message spell.
IE. Two spellcasters are imprisoned without any gear. The lock on their cell is VERY impressive and magical. They decide to try casting Knock jointly. (Which BTW in my world does not have the massive audible sound, which is just silly.) They are not rushed or in combat so success is assured.
Spells need not be identical to be jointly cast either. One might be casting fireball and the other casting Fire Bolt. But the magic will intertwine and strengthen both spells.
Casting VERY different spells jointly will have, unpredictable, results. Some perhaps very good, some quite horrible. (IE. One caster casts Fireball and the other tries to jointly cast Ice Storm for whatever reason.) Think of this as the "test your luck" teamwork game.
#3 Spells are repeatable for improved chances of success.
If you cast a spell and it fails you can try again. The second casting has an increased chance of success due to what you have learned from your failure. Also spending time reviewing your first casting during a short rest will also add an additional positive modifier to your chances.
#4 These rules are meant to improve the roleplay, not hinder spellcasters.
In some ways you're now more powerful, on some ways you're a little weaker. I see the game as a story. Written by the characters. Which means if I put up barriers that seem logical in the world/story it is also my job to assist you in working around those barriers.