Alright, we pretty much all seem to agree that Improved Familiar makes sense as a feat for Wizards and Warlocks, but none of us really seem to agree on how it should actually look. Since this is my thread, I thought I should throw my own two coins into the fray.
As I keep saying, I think the Improved Familiar feat should function by allowing both Wizards and Warlocks to have more familiar options. That was its key purpose in 3e, and I think it should remain its key purpose in 5e. But, before we do that, let's take a look at what the Familiar options actually look like in 5e.
So. The big difference between Wizards and Warlocks is that warlocks get a slightly more combat capable familiar, as well as the option for limitless telepathic communication with it. The Improved Familiar feat will, inevitably, step on those toes a little, but it's still requiring a wizard to make a sacrifice - remember, feats take the place of an Ability Score Increase, and a class doesn't get very many of those - for something that the Warlock gets for free. And, personally, I consider that fair.
Which means, just off the top of my head, this is how I'd do Improved Familiar as a feat.
Improved Familiar
Prerequisite: Ability to cast Find Familiar, 5th level
Your strength and studies have unlocked a more powerful version of the familiar-binding ritual, allowing you to gain the aid of a stronger being than the common mage could hope to summon.
Effect:
- You replace the normal Familiar list. Instead, when you cast Find Familiar, you can bind any Tiny or Small creature of the Beast, Monstrosity, Aberration, Undead, Construct, Fey, Celestial or Fiend type with a Challenge Rating of 1 or lower. Beasts you take as familiars change their type to your choice of Celestial, Fey, Fiend, Aberration, Construct or Undead.
- When you take the Attack action, you can forgo one of your own attacks to allow your familiar to make one attack of its own. If you already had this ability, then you can now spend your Reaction to allow your familiar to make its attack instead.
- When you reach level 10, your Familiar can be a creature of the appropriate type with a challenge rating of 2 or less.
- When you reach level 15, your Familiar can be a creature of the appropriate type with a challenge rating of 3 or less.
Fun fact is, winter wolves and worgs are only Challenge 1/2, but they're Large creatures. Hippogriffs, another "familiar steed" you could get through Improved Familiar back in 3.5, are only Challenge 1. So, for added fun...
Familiar Steed
Prerequisite: Ability to cast Find Familiar, must have Improved Familiar
Your strength has grown to the point you can command larger creatures to do your bidding, allowing you to ride into battle on their back.
Effect:
- You can now take a Large or smaller creature of the appropriate Type and Challenge Level as your familiar.
- Because of its bond to you, this creature will allow you to ride it as a steed.
- The magical nature of your bond means you have Advantage on any riding-related checks made whilst mounted upon your familiar.