In the context of character concepts, a frequent suggestion is that the DM invent a feat to cover the related capability. Leaving aside my opinion that feats should be much more available than they are, there is space for a huge amount of innovation to enable specific character concepts. I'm going to invent a few such feats here, with a short explanation of the purpose and meaning of each - hopefully reddit will help me find any exploitable loopholes that I didn't notice!

Aikido

While not wearing medium or heavy armor, and while having at least one free hand, you gain the following benefits:

• You may use a Wisdom (Insight) check instead of the usual Strength (Athletics) check when attempting a grapple or shove.
• When a humanoid enemy no more than one size larger than you attempts to grapple, shove, or charge you, you may use your reaction to shove with advantage, possibly preventing them from their attempt.
• If you successfully shove an enemy who has already moved any distance this turn (voluntarily or not), you may knock them prone and push them away.

This enables a somewhat more accurate approach to grappling, and makes it viable for many more characters (most importantly the monk). The cross-ability skill check seems appropriate - making it dependent on Dex(Acro) would make it too convenient, and doesn't seem as realistic to me. A lot of people on reddit are concerned that allowing dex-grappling is too powerful on its own - in my opinion, the real question to ask is "does opening grappling up to dexterity-based builds cause any problems?" I think it will not, but you shouldn't offer the feat if you're concerned about it.

Combat Reflexes

At the end of any turn in which you used your reaction, you may attempt a Dexterity (Acrobatics) or Wisdom (Insight) check to regain your reaction. This check has an initial DC of 5, but that DC increases by 5 for each time you have succeeded since the beginning of your last turn.

A refresh of an old standby. This is a substantial effectiveness boost for defender-style characters, who were already in a good place. To keep it under control, I made it difficult to predict in advance how many reactions you would have, and I made it basically useless against a single target.

Spell Focus

Requires: Ability to cast at least one spell.

Choose one spell you know or have prepared of first level or higher. You are able to cast it without any somatic or verbal components it normally requires, and you may use your own mind as a holy symbol or arcane focus. Your save DC is considered to be 1 higher when casting this spell. An observer can tell that you are casting the spell at by succeeding on a Wisdom (Perception) check opposed by your Charisma (Deception). You may change the selected spell whenever you gain a level.

This opens up illusions and charms to be used without revealing their usage, which can open up a huge amount of tactical options for an illusionist. It also allows somebody with both hands full to still make use of a certain spell in combat - an EK with the Shield spell, for example.

Mystic Studies

Requires: Ability to cast at least one spell of 2nd level or higher.

Select any one cantrip and one spell, which must be of lower level than the highest spell slot you possess. You may cast both as if they belong to one of your classes. Each time you gain a level, you may change the selected spell.

This opens some flexibility up to casters without dramatically increasing their power. Since it's otherwise comparable to a major bard class feature, I had to restrict it by level - that should also reduce the potential for abuse, since the truly crazy spells are above level 5. It might be reasonable to allow one spell of each level from 1-5 instead, but not to allow changing them once they are selected.

Blindfighting

• Gain 1 Wisdom or Dexterity
• Gain 5' of hearing-based blindsight.
• When something makes a sharp or loud noise, you can roughly judge the distance and direction it came from - if it came from within 60 feet, you can determine the exact location.
• You may take a bonus action to make a sharp noise and listen to its response - this allows you to learn the locations of any walls, dropoffs, or other major features within 60 feet, and any medium or larger characters within 30 feet. In combat, make this check with disadvantage except during a surprise round.

I have been playing in an Underdark campaign, and basically every character has to be a warlock or a race with darkvision, or spend all their time being led by the hand. This feat attempts to give more options for adventures and campaigns that tend to have a lot of darkness. It's also helpful alongside the spell 'Darkness' and against invisible enemies. The feat was pretty niche, so I added the wisdom/dexterity bonus to keep it viable.

Battlemage

• You no longer have disadvantage when casting ranged attack spells while adjacent to an enemy.
• You may treat a weapon you are wielding as an arcane focus or holy symbol.
• As a bonus action, you may make an attack or shove against the target of a successful spell with range 'touch'.

This feat focuses on the underserved melee caster. It resolves some of the annoyance of focus-juggling, and helps you be more effective at using spells from melee-range. The last feature allows non-EK casters to get some use out of their bonus action.

Skill Focus

Choose one skill in which you are proficient - your proficiency bonus is doubled when it is applied to ability checks in that skill. Gain advantage on one ability check with this skill - you may use this ability again after finishing a short or long rest. You may select this feat multiple times, choosing a different skill each time.

This is uncomplicated - it won't make you a rogue, but it will let you excel at one thing, which seems like the definition of a feat. It also takes away the need for so many builds to take one level of rogue, which I consider a plus.