Negative Space
I can't emphasize enough how important of a skill this is. When you are developing, the proper skill and utilization of negative space will allow you immense flexibility by taking aspects from one thing and incorporating it into what you're making without it being a complete rip-off.
Here is how I define Negative Space:
"Think in negative space. The warlock's spell slot resource is actually a short-rest spell point system, except the designers decided to "summarize" how many spell points you had at the given level and made them into spell slots that then scaled with the warlock. Imagine if they had spell points equal to their level that restored on a short rest that followed the rules of spell point cost (DMG, 288). While the power curve is wonky, its very close.
Another negative space is the paladin lay on hands. It's actually a per-level ability score modifier healing ability, like the Druid's Balm of the Summer Court is per level healing. The designers decided to instead summarize it as a 5, so the healing felt more impactful when used.
Thinking with negative space is by far the most powerful tool you can use to create new features, but is also the hardest skill to learn. This will help you take the feature of one class and splice it into yours without anyone ever knowing."
I created this concept when I started realizing what I was doing by looking at other people's work and using it for my own. Suddenly I went from struggling to create my own features to creating very original ideas, simply by parsing "negative space" from its design. It took me a few months to really develop this skill, and once I did it has become my strong arm. The level and speed at which you develop your design skills begins to skyrocket. I'm still learning new negative spaces to work with.
Example Designs
Okay, so enough talk, now let's do the walk. How do we create a new feature? Let's say you have a martial class or subclass but you don't want to use Extra Attack, but you have the "budget" for it. What is an extra attack? At 5th level most players will have around a +3 to their modifier for an attack (Spells, weapons, or otherwise). A d8 (die roll average: 4) is the average for martial weapons, and a d6 (die roll average: 3) is the average for simple weapons. Let's say we're making something rogue-ish, like a hunter taking an advantage of a weak point. We'll be safe and use the expected damage of a simple weapon with the expected attack score modifier of +3. I'll use an idea I've already come up with:
Spot Weakness
5th level Hunter feature
Once on each of your turns when you hit a creature with a weapon attack, you can cause the attack to deal an extra 2d6 as damage of the same type dealt by the weapon to the target.
(Wording of this feature borrowed by Cleric, War Domain)
Let's dissect this. "Once on each of your turns" is basically an additional attack because you get this in addition to your normal attack. It also defines on your turn, so this doesn't apply to anything like opportunity attacks (Which it shouldn't, since extra attack doesn't affect those). The next part is an "extra 2d6" damage. The average of a d6 is 3, and the player is expected to have around a +3 at this level. Thus, "2d6" is really the average of a d6 weapon + the expected damage modifier for the attack.
Suddenly we have an entirely new extra attack feature without stepping out of line in terms of expected damage. This can also be used to control player damage for things like multiclassing; it creates a vacuum in the class (vacuum meaning it can't be used elsewhere). By having this vacuum we can safely improve upon it without it affecting too much power for multiclassing. Subclass features or core class features could potentially improve the amount of damage you deal, such as increasing the die size to a d8 or adding more dice. Alternatively the damage type can be changed, potentially allowing a player to take advantage of vulnerability.
Bonus "Proficiency"
Okay. Let's look at another negative space: Proficiency. Let's say the class had a class die that scaled with its levels from a d4 to a d12. If a class did this, look at the proficiency bonus at the same levels it improves in die size. It should match the levels your proficiency bonus improves. This is because proficiency bonus is actually a 1d4-1d12 summarized as its average die size (Rounded down).
Now I'll think of a new example I've never tried using this space:
So let's say we have a feature that grants proficiency in all saves. We could use that feature, but what if we used our class die? "You add your class die to all saves" is pretty exciting (Well to me it is. I love rolling more dice). What if you could, at will, give this dice as a reaction to someone attacking another target within 5 feet, adding it to their AC? Like a parry? That also sounds exciting. You're always reacting to the action each and every turn, giving you even more to do.
Conclusion
The power of Negative Space is extremely powerful. Learning how to use it with spells and spell-like features will allow you to make new classes with new features that seem completely original but in fact uses features from elsewhere.