The DMG provides the following stat-increase items:
Each of those set a stat to "19" unless it's already above.
There no equivalents for Wisdom, Charisma, and Dexterity. May I simply homebrew such items, or am I overlooking a huge balance issue?
Short answer: Dexterity is too powerful while Wisdom and Charisma are more difficult to justify changing. You can homebrew them just as you can homebrew anything, but you might be violating balance concerns for Dexterity if you do.
Okay, I admit that wasn't very short, but it is a complex answer. I'll try to tackle them more in-depth now.
First, why aren't those items in play?
The Strength- and Constitution-setting items seem fairly straightforward, increasing the strength and endurance of the body. Intelligence also fits this, being the closest representation to pure brain power. Dexterity and Wisdom don't seem to fit this mold as well because they are about the flexibility (physical or mental) to use the power that one does have. Charisma could probably work just fine, being like the Blarney stone or similar that causes someone to just have a forceful personality. However, there is justification that Dex and Wis could also be used as some creatures are inherently more dexterous or wise than others.
Second, can you homebrew them.
Same as my short answer, a DM can homebrew anything, but it raises concern with...
Third, is it balanced to do so?
This is a tough answer, and the answer is dependent on different campaigns. Let's break down the current abilities for a bit...
Strength items. Strength is something that is useful for everyone (more carrying capacity, mainly), but often is only needed to a certain point. Those who would use Strength as a primary stat likely have it higher or the same already and attacking with Strength is less-than stellar for most other builds as they can attack with their main stat (spells, Dex attacks, basically anything except Con gets offensive techniques), basically only being useful for spellcasters who are stuck in melee, need to damage, and don't have any saving throw cantrips. It can be used for AC, but you only need 15 for that and most who would desire that already are striving for it. Out of combat, Strength is very useful for hauling stuff, increased jumping, and a variety of ability checks. This is really the pillar where the gauntlets shine, but the effect isn't tremendous. The most important part is the fact that the wizard who dumped Strength no longer has to worry about failing DC 5 checks, mitigating a great amount of danger that would be ridiculous to fail on.
Intelligence is an odd choice in that it has precious few combat applications (except for wizards!), but is also fantastic for exploration through its number of knowledge skills and Investigation.
Constitution is the opposite of Intelligence, with the vast majority of its improvements part of combat. HP and Con saves (including concentration) can play a great part in survival, while Con checks have sparse but important uses (succeeding on Dash checks in chases, not passing out from liquor, etc.).
From the above we can see the three paradigms.
Intelligence mainly favors out-of-combat applications and is uncommon, Constitution mainly favors combat and is rare, while Strength has a little of both but not enough to get past uncommon.
For the more versatile abilities ...
Charisma becomes fairly easy to judge. It is to the social pillar what Intelligence is to the exploration, extremely useful in that sphere but limited in-combat. Thus, the Charisma item could safely be set as uncommon.
Wisdom is harder. It has just as many skills as Intelligence (5, more than Charisma), including one of the most important for exploration and combat (Perception) and the most important non-Charisma skill for social (Insight). Already, it is riding up in rare territory due to being useful for skills in every pillar. Further, it is a major saving throw (one of the key reasons why Constitution is a rare item) that corresponds to debilitating effects that can outright remove the user from combat. That is a really potent ability. I would say Wisdom is at least very rare, it is just that powerful.
Dexterity is similar to Wisdom. Stealth is incredibly useful for both exploration and combat, Acrobatics provides one of the primary reasons why non-Strength users use the gauntlets (escaping grapples and the like), and Sleight of Hand can play a key role in both exploration and social pillars. Further, it can be used to increase the weapon capabilities of non-primary users, another key point of Strength. We are already a solid rare, maybe getting into very rare, as it is an ability that takes most of the powers of Strength and then powerful uses in everything in other ways. Finally, it also gives AC, but this one is even more worrisome than Strength. With that ability, you needed armor proficiency to really make use of it, but Dexterity can be used for AC by everyone. Barbarians can get a dramatic rise out of it if they have been focusing in Con, while both wizards and sorcerers can suddenly find ASIs able to be devoted to feats like War Caster rather than buffing AC. Even strength-users will love having a decent AC when they are caught otherwise naked! On top of all this, it is also a major save (admittedly more for damage than dangerous effects). Definitely very rare, maybe getting to legendary.
Fifth Edition emphazises the role of the DM as an arbiter but also as a game designer. This quote from the DMG explains why the list of magic items shouldn't be seen as exhaustive (p. 284, emphasis mine):
Creating a Magic Item
The magic items in chapter 7, “Treasure,” are but a few of the magic treasures that characters can discover during their adventures. If your players are seasoned veterans and you want to surprise them, you can either modify an existing item or come up with something new.
The chapter then continues with advice on how to determine the strength of the created item.
That way, nothing stops a creative DM from turning those Gauntlets of Ogre Power into a Brooch of Nimble Fingers or a Thurible of the Wise, and have it boost the relevant stat.
Other responders have examined the relative value of each stat in 5e. However, there could be other reasons for not including Cha and Wis items, namely balance for multiclassing spellcasters.
The reason I clicked on this thread was to see how powerful a Warlock 2 dip could be. With a Charisma equivalent of Gauntlets of Ogre Power, or Belt of X Giant Strength, a character could use eldritch blast, perhaps the best damaging cantrip in the game, almost as powerfully as a Warlock of the same (combined) level(s).
Currently, characters multi-classing into the wizard class could pick up a magic item that allows them to use the class at close to maximum effectiveness thanks to a magic item. If casters could be reassured they could dump their casting stat (or keep it to the minimum for multi classing) and obtain a magic item then any optimization focused build would involve raising Con and Dex as high as possible, taking any relevant feats, while ignoring increases to their casting statistic.
It would completely alter the feel of magic using classes, and specifically multiclassing, in 5e.
