As my starting point, we have Mage Armor. We know it. We love it. It's a great low level spell. Can we scale it up while keeping the balance of usefulness and cost? If mage armor is a level 1 spell that gives 13+Dex armor class, could you ratchet up the armor class as well as the casting level by four and make a 5th level Heavy Mage Armor spell that granted 17+Dex armor class?

A few questions on my mind:

1. Should it require concentration? That would stop it from stacking with Shield of Faith for instance, but do we need to stop it from stacking?
2. Should the range remain touch or should it be self?
3. Should we continue to treat it as light armor where the full dexterity bonus applies? I feel like if it didn't, there would not be much use for it.
4. How can we skew the balance for high dexterity characters? 20 dexterity = 22 armor class plus a shield = 24 ac
5. Is 17+dex armor class appropriate for a 5th level spell?

18 AC on a spell using a Dex bonus (of any magnitude) is a hard line in the sand. 17 AC without a Dex bonus is another hard line. Bounded accuracy and such. Heavy armors come at the cost of currency, availability, and disadvantages (beyond just Stealth).

Bottom line: If the spell allows a high Dex character to exceed that of Plate (18), you're invalidating all armor. If the spell allows a zero-to-mid Dex character to exceed 17, you're invalidating: armor, Tortles, and Warforged.

I think Mage Armour is good as is. It's not as good as full armour but then it's not supposed to be - to compensate for being at range throwing spells and the life-saving option of Shield spell. It's also so it doesn't become too much when using as a Bladesinger who gets to add their Int score to AC. There's also magic items like bracers of defense (+2 ac) and the cloak/ring of protection (+1 ac and saving throws). If they get shield proficiency then that's another +2, up to +5 with magic shields. 

Now throw in defensive spells like Blur, Mirror Image, Fire Shield, Absorb Elements, Shield, Haste, and more which all have protections... Wizards can be very surprising tanks. 

With magic items and spells Wizards, as an example, can achieve extremely high AC. My 17th level Bladesinger is capable of AC 40 - no homebrew., even if only for a round. 

The spell is fine.

There's already precendent with Barkskin and Stoneskin. Just follow that template and make a 1 hour (concentration) 3rd level transmutation spell called Steelskin that sits in between them. 19 AC (following the pattern of Mage Armor improving on an armor category by 1 point), adds 70 lbs to your carrying weight and gives disadvantage on stealth.

Story bits: It's a relatively obscure wizard spell along the lines of Tenser's Transformation. Bladesingers have no interest in it since it weighs them down and few wizards invest in enough strength to make it useful. Druids can't use it due to the taboo against metal armor. Maybe it's a dwarven invention, which would explain why a wizard would come up with a spell like this in the first place and why it's not better known.

Disclaimer: heavily inspired by the Iron Flesh pyromancy from Dark Souls.

An interesting alternative I've been toying with: Battlemage Armor/Conjure Armor.

Third or fourth-level spell, cast in two ways.  The first is a one-hour ritual which binds a suit of nonmagical armor to the ring used as the spell's material component (ring is made of the same material, in the same style, as the target armor).  Once that's done, the wizard can cast the spell again to summon the armor onto their body directly.  Armor bound to the wizard in this way ignores the wizard's lack of proficiency - they're proficient in their summoned armor while it's summoned with the spell.  They can dismiss the armor again as an action.

This allows the wizard to use whatever nonmagical armor they find, possibly inheriting the rest of the party's hand-me-downs as everyone else gets into cool magical outerwear, but it respects the normal armor rules.  A wizard that wants to wear heavy armor needs to have enough strength to do so (or be a dorf), and a wizard that wants to get the most out of medium armor would need 14 DX to get that +2 bonus.  They also don't get to bind magical armor, so no shenanery that way and no stacking +1s.  But conjuring up armor out of the aether is still kinda cool, and it allows the wizard the advantage of being able to get into and out of their bound armor quickly - which can be a very big deal in some situations.

How about less of a dial-up and more of an alternative? Something like AC15 +Dex (max 2) as an optional spell for lower Dex casters as a level 2 or 3 spell.