FormerlyHemlock
Hero
We can talk about Paladins, but only as they relate to the use of wizards. Good point that Warcaster + Booming Blade can be used for a reaction opportunity attack. From the standpoint of a Wizard, I don't see using reactions for opportunity attacks very much, but for a tank-style character who does, yeah, using booming blade at a moment when the secondary effect is automatically going to trigger is pretty fantastic. That said, I'm no expert of Tank-style characters, but it seems to me that magic-initiate and warcaster is a massive investment to make.
It's more likely to be Warcaster + multiclassing to a class a spellcaster wants anyway, e.g. Paladin/Wild Sorcerer. You gain access to Booming Blade at the same time you're gaining access to Shield, Expeditious Retreat, and Tides of Chaos (awesome for tanks--if casting Shield accidentally produces a Fireball as a byproduct, it is the enemy, not your allies, who are going to suffer most from it), and extra spell points/slots for casting awesome Paladin spells like Wrathful Smite and Aura of Vitality.
Life Cleric 1/Enchanter X is also a fun tank build which can make good use of Warcaster + Booming Blade, if you're not using it on Instinctive Charm this round.