For Wish Passing for passing's sake as well as the HP that the poke can pass, I'd recommend Sylveon.
Sylveon@Leftovers
Ability: Pixilate
Nature: Bold
EVs: 252 HP / 252 S Def / 4 Sp Atk
Protect
Moon Blast
Toxic
Wish
Following up on this, I think I've got a Primarily Defensive core happening.
Sylveon@Leftovers
Ability: Pixilate
Nature: Bold/Calm
EVs: 252 HP / 240 S Def / 16 Spd
Substitute
Moon Blast
Toxic
Wish
A fairly generic Sylveon build, soaks up special hits, heals them off with Wish and likely to go for the Protect the next round. This makes it easier for Aegislash to come in safety and use King's Shield. Moon Blast is its best attack until Pokebank so Pixilate starts being some worth. Toxic is for chipping away at opponent's health and making them easier prey for the other two parts of this trio. Bold nature so if it gets caught taking a physical move in its hide it may be likely to survive, otherwise we'll go calm.
16 Spd is for out speeding Florges, other Sylveons, non-invested Skarmory and Jellicent (avoiding taunt), Politoed, Scizor (may become a thing with its Mega) and T-Tar.
Synergy: Resists Dark types for Aegislash, taking Neutral from Ghost/Grass that would seek to hurt Aegislash/Rotom-W. Can take Fire/Ground for neutral for Aegislash but that's better left to Rotom-W. Provides an immunity from Dragons which is golden for every team. Neutral damage isn't so bad, as long as Sylveon isn't getting hit on its physically side, healing off damage with Wish.
Aegislash@Leftovers
Ability: Stance Change
Nature: Brave
EVs: 252 HP / 252 Atk / 4 Def
IVs: 0 Spd
Substitute
King's Shield
Iron Head
Sacred Sword
You may notice there's no Shadow Sneak here, personally pokes weak enough to be picked off by it, I imagine would switch and then die to SR upon coming back in. It seems too risky if they live and catch Aegislash in Blade Form. Running 0 Spd Ivs and a Brave nature further seeks to ensure Aegislash moves last. Substitute is so Aegislash can hide behind it and try to get off its Iron Head/Sacred Sword STAB while not so easily being caught in Blade Form. Seeing as upon switching in its in Shield Form it doesn't look to hard to get it in, though it is relying on Sylveon for those Wish passes after Subs.
Will-O-Wisp will cripple Aegislash massively so Heal Bell on Sylveon once it becomes legal is a must, otherwise a poke that can run it or Aromatherapy.
Synergy: Is immune to Poison types that may attempt to hit Sylveon, as well as resisting Steal. Also resists Grass for Rotom-W. Provides a Normal/Fighting immunity on the team.
Rotom-W@Leftovers
Ability: Levitate
Nature: Calm
EVs: 248 HP / 28 SpA / 232 SpD
Volt Switch
Hydro Pump
Will-O-Wisp
Pain Split
It's the Specially Defensive Rotom-W set! Volt Switch keeps up momentum while the opponent will possibly switch on the switch-in, racking up hazard damage. Hydro Pump is strong STAB even with minimum investment. Will-O-Wisp further seeks to cripple pokes when Sylveon's not able to, a poke running Thunder Wave on this team would also work well here. Pain Split allows Rotom-W to heal off its opponents.
Synergy: Rotom-W provides an immunity to Ground, laughing off EQ for Aegislash. Also is immune to Fire types and can take Ghost/Dark moves for Aegislash. Provides another steel resistance for Sylveon, will still taking neutral from Poison attacks.
Overall the core synergy isn't amazing, Aegislash still fears Ghost moves that can't be sponged by Sylveon/Rotom-W. So perhaps a Dark/Normal type poke is in need for this team, I'd be thinking something like Assault Vest-T-Tar, Mega-T-Tar, possibly Hydreigon (as it and Aegislash have perfect synergy) or Porygon-Z.
This core values SR on the field to take advantage of switches and also would love Heal Bell/Aromatherapy to ensure everyone's longevity. To me, this is Defense Core, with Sylveon also being 1/2 of a Support Core, providing Wish for everyone.