One category of feats that I feel needs some love is what I think of as cross-class feats; feats that appropriate a class feature from one class for use by other classes. There are a few good examples in the book, like Martial Adept, but the complexity of applying class features out of context safely has mostly been implemented via multi-classing. I've explained my dislike of multiclassing a few times (moves the power-curve breakpoints, obviates whole sets of feats, overly costly for casters), but I prefer to address that type of hybrid concept using archetypes and supporting feats. So here I've added a few feats of that type that have come up in my games as players try to explore interesting concepts and make them into contributing characters.
I'll group them by the class they're extracted from.
The warlock class is very modular - there aren't really any class features (aside from spellcasting) that can't be extracted into individual feats.
You may choose one of the Otherworldly Patrons available to warlocks; immediately gain the feature it grants at 1st level. The features available to warlocks at 6th, 10th, and 14th level become available to you at 7th, 11th, and 17th level, respectively.
This feat represents making a (possibly secret) pact with one of the powerful beings you have encountered on your travels. You may choose one of the Pact Boons available to warlocks, though you may only choose Pact of the Tome if you have the ability to cast spells - the spells it allows you to cast are considered spells of that class, rather than warlock spells.
Select one eldritch invocation as if you had warlock levels equal to character levels (other constraints must be satisfied normally). This feat can be taken multiple times - each time, you must select different invocation.
Unlike the warlock, the Druid is built around two very strong, scaling abilities: spellcasting and wildshape. Both of these are far too strong to be feats, and are better addressed through class archetypes. But wildshape can be diluted sufficiently to become a feat, though it won't be very combat-relevant often.
Select two beasts that you have encountered, and which have CR no higher than 1/4. You may shift (as wildshape) into each of those beasts once, for a maximum of ten minutes; you regain this ability after each short rest. The beasts that you choose are subject to the same restrictions as the wildshape druid class ability (though based on your character level rather than on your druid level), and may be re-selected any time you gain a level.
I've already extracted Magical Secrets as a feat elsewhere, but there are a few other reasonable choices here.
You may use your bonus action to choose one creature other than yourself within 30 feet of you who can hear you. That creature gains one Rakish Inspiration die, a d6. Once within the next minute, the creature can roll the die and add the number rolled to one ability check, attack roll, or saving throw it makes (it may choose to do so after the d20 is already rolled).
You may use this feature once, and you regain the ability after finishing a short rest. You gain a second die at level 5, a third at level 11, and a fourth at level 17.
You can add half your proficiency bonus (rounded down) to any ability check you make that doesn't already include your proficiency bonus.
The main ability that would be useful to other classes is Rage, but that's a bit too strong for a feat as implemented - we can reduce its impact by allowing only one per day.
On your turn, you can enter a rage as a bonus action. While raging, you gain the following benefits if you aren't wearing heavy armor:
You may not cast or concentrate on spells while raging. Your rage lasts for 1 minute. It ends early if you are knocked unconscious or if your turn ends and you haven't attacked a hostile creature since your last turn. You can also end your rage on your turn as a bonus action.
You can only rage once, and you regain this ability after finishing a long rest.