How would the game change if I removed spell lists? Essentially, every spell would be on every spell list.
I know this would lead to balance issues with wizard's Spell Mastery and spells such as Healing Word. Are there any other spells that would cause issues?
Are there any other potential balance problems that every spell being on every spell list would cause?
For this answer, I'm going to try to focus on broad-strokes balance issues, rather than particular broken combos.
Druids and clerics are both prepared casters who use WIS as their casting stat, but have very different spell lists. If you remove the spell list differences, these classes' abilities will become very similar. Balance-wise, this means everyone can optimize based on which class has the best non-spellcasting features, instead of having to factor in the spell list.
In particular…
It's by design that wizards are very bad at healing, and clerics are bad at blowing things up. This is a balancing factor that's meant to counteract the 3e-era problem of "the best party is always four wizards" and incentivize playing a mixture of classes. In base 5e, four wizards will have great DPS and crowd control, but no healing and very little defense. With this change, that's no longer a concern.
Spontaneous casters won't get as much of a benefit from this, since even if they have access to an order of magnitude more spells, they can't learn any more. Prepared casters, on the other hand, can take advantage of this new panoply of options at every rest. (And of course classes without spells get no benefit at all: fighters and rogues get left out in the cold.)
Which also means…
A prepared caster like a cleric now has the option of preparing any spell in the entire game at a long rest. That's a lot of options! The player has gained a whole bunch of extra power, but only if they can OOC think through all those options…and this comes up anew every day. This puts more burden on the player's OOC planning and decision-making skills to take advantage of the expanded list.
But also…
Spontaneous casters, on the other hand, still have a very limited number of spells known. With all the spells in the entire game to choose from, it's likely all of those choices will get filled in by the objectively best spells.
Currently, a ranger might take a couple of "mandatory" spells that are the best on their list, and then fill out their other choices with fun or flavorful effects that aren't as potent. But with this change, the opportunity cost for choosing those fun spells becomes much higher—because you're taking them instead of the best spells from the wizard list, the cleric list, and so on. While prepared casters may end up with too many choices to make, spontaneous casters are likely to end up with too few!
This is a far bigger change that it may appear on the surface. The balance of many class features is built around the spell available to that class. It also removes most of the distinct between spellcasting classes and significantly changes the balance discussion.
Ignoring the balance concerns for a moment, this change removes all the flavour and "feel" of the different spellcasting classes. Much of the distinction between a Wizard and a Cleric is found in their spell list. Remove that and there is far less incentive to have a variety of classes and instead just pick the class that lets you learn/cast the most spells per level.
There are multiple class features or subclasses whose entire premise is based on giving access to spells that class wouldn't normal have access to. Some examples:
By 10th level, you have plundered magical knowledge from a wide spectrum of disciplines. Choose two spells from any classes, including this one.
Your link to the divine allows you to learn spells normally associated with the cleric class. When your Spellcasting feature lets you learn a sorcerer cantrip or a sorcerer spell of 1st level or higher, you can choose the new spell from the cleric spell list or the sorcerer spell list.
These are just a few examples of features made redundant by your change.
On top of weakening specific features this change largely makes multi-classing for spellcasters redundant. Gaining access to powerful combinations at the cost of high level spell is one of the primary motivations for multi-classing. Taking that away means that most players will simply pick the class that gives the highest number of spells.
While making some features redundant, it also makes some other features far stronger than they currently are. Typically by allowing access to strong combinations of spell and feature that typically requires multiclassing to achieve.
Some features that majorly benefit from this change:
This is a change that impacts so many classes at such a fundamental level that it is difficult to foresee every powerful or redundant interaction. If allowed during play it is likely players will regularly find new combinations that break the game in new ways.
