Curious to know if we might see some of those classic "reactive" armor enchantments, you know the ones where someone hits you and get an electric shock, or get a fireball shot at them

Hope not.

"classic"? I don't recall there being any "reactive" armor enchantments. Although I've only played Morrowind and Oblivion so I can't say if they were in prior games to those.

I would be fine if Fire Shield or Frost Shield spells did damage to your opponents when they hit you, but really I'm kind of meh on they whole "reactive armor enchantments"

Well such feature wasn't "classical" but there was other similar effects
Shock spell in Arena Creates a charge of electricity to cover the caster's body. The next creature touched will take 1-35 points of electrical damage to health, plus an additional 1-5 points for every level of the caster.

Aura spells in Daggerfall
Trough not all of them work, but they should affect with spell effect on actors thats surround caster

Elemental Shields do appropriate elemental damage to enemy at close distance in Morrowind
Unlike in Oblivion elemental shields doesn't add to armor rating instead they do damage in close range, for shielding was used Shield spell effect, what one become useless in Oblivion.

Reflect Damage in Oblivion
Thats was good spell effect but need to be capped on 25-50% for custom spells

On Struck enchant option that's trigger spell effect when struck on armor and shields as well at parry for weapons, will be nice option for mages.

All of such effects can be achievable in Oblivion with mods since can be scripted.

I would like to have the "cast on use" option back. Reactive could work, but I won't hold my breath.

Yes return "cast on use" options will be awesome just need to make it have cast time like spells have or/and have chance of miscast depend on character enchant skill and intelligence, since logically need actually to know how use enchanted item in such way.

What the Stalker said