The most likely candidates right now, In my Opinion:
Nightblade
1. Assassin's Blade / Killer's Blade / Impale
2. Veiled Strike / Surprise Attack / Concealed Weapon
3. Teleport Strike / Lotus Fan / Ambush
4. The new version of Haste that they're adding
All of these abilities (minus #4) currently function based on weapon critical strike chance (though they also scale with magicka/spell power). The new Haste is rumored to add a spectral bow attack, which sounds like it could be stamina based as well.
Templar
1. Puncturing Strikes / Biting Jabs / Puncturing Sweep
2. Piercing Javelin / Binding Javelin / Aurora Javelin
3. Focused Charge / Explosive Charge / Toppling Charge
4. Spear Shards / Luminous Shards / Blazing Spear
Again, #1-3 are all currently based on spellpower/magicka but weapon critical chance, making them most likely to get a stamina morph. #4 is currently not based on weapon critical chance, but it does fit in with the theme of the first three.
Dragonknight
1. Fiery Grip / Empowering Chains / Extended Chains
2. Searing Strike / Unstable Flame / Burning Embers
3. Lava Whip / Molten Whip / Flame Lash
4. Stone Fist / Obsidian Shard / Stone Giant
Once more, the first 3 abilities are based on weapon critical already. Stone fist seems more like a physical ability than a spell to me, but it's thrown in their just to keep a #4.
Sorceror
2. Lightning Form / Thundering Presence / Boundless Storm
3. Surge / Power Surge / Critical Surge
4. Unstable Familiar / Volatile Familiar / Unstable Clannfear
5. Winged Twilight / Restoring Twilight / Twilight Matriarch
Lightning form and Surge are both heavily utilized by melee/stamina-centric sorceror builds currently, with critical surge being a cornerstone of melee sorc builds. With the pet changes coming up, the two summons seem most likely to get a stamina morph to me, so that they can be equally utilized by stamina/melee sorcerors.
Keep in mind that only one of the two morphs for each ability will be getting changed to stamina, so it's an option for melee combatants if they choose that morph.