Introduction
Magic items are a lot of fun and allow for many new options for
players. It can also be one of those things that make a game
memorable. However, certain classes get more attention than others.
In particular, rangers and non-magic classes with magic options are
left out. For this reason, I'll be looking at the Eldritch Knight
specifically to provide some magic items that benefit the class.
How to Limit it?
Many items in the new addition require attunement by a particular
class. This helps limit who can use the magic item and ensures that
the intended class gets the benefit. Since we are making this new
item, we need to consider if we will do the same. A staff is not as
useful to an Eldritch Knight as a pike or longsword is. It's not very
common for the normal magic classes to have access to these weapons
so we can tie it to the proficiency needed for the item. There is
still a chance that someone will take a feat to gain proficiency with
the class of weapons. If this bothers you, you can make being an
Eldritch Knight a requirement. I'd be wary of this though because the
rogue also has a magic option and I wouldn't want to cut them out. At
the same time, even if a wizard has the required proficiency, the
item is far more useful to an Eldritch Knight as a spear if their is
a weapon bonus.
An option is to provide certain bonuses to an Eldritch Knight that
are not provided to other classes using the item. Most typically,
this is because it adds to a class feature that the other classes
don't have (extra hitpoints when using second wind, for example).
What Kind of Items?
Typically, the most common items I've seen in this case is taking a
normal staff that any spellcaster could attune to or only specific
ones (such as the staff of the magi) and switching it over to
Eldritch Knights for attunement with a different weapon. For example,
take a staff of the magi, change the attunement to Eldritch Knight
and swap staff to spear. You can also take the effects of a robe or
similar magic items not availible to an Elritch Knight and either
make an exception, create a new item that they can use (otherwise
identical), or add the property to a different item (add the
properties of the robe of the archmagi to a suit of +3 plate armour).
The results are potentially extremely powerful, but the items we are
taking the effects from are already powerful.
There is, however, more you can do. I would recommend giving another
option for the charges that helps the martial side of the class.
Similar to some other items that already exist, allowing the Eldritch
Knight to spend some capped number of charges (max number of 3) to
do extra damage (1D6 or 1D8 per spell slot from a staff of fire, for
example). Such a feature would be far less useful to normal
spellcasters but extremely beneficial to the multiattack features of
the Eldritch Knight. You can also, or instead, allow the user to cast
a fireball centred on themselves to which they are immune at the same
time as they take the attack action. Items that boost the slot level
used, grant resistance against some type of damage (including spell),
or allow spells that they usually would not have access to (being
stuck at 4th level spells at the most) are all good
contenders as well. You can also allow more uses of a particular
unique class feature (extra use of action surge or second wind) but
I'd be more careful about this kind of thing than the above. Applying
a modifier to the second wind feature seems to be relatively safe
from the games I've played while also being a significant benefit.