A wonderful example of this is the Wand of the War Wizard, (p.212, DMG).
uncommon (+1), rare (+2) or very rare (+3)
While holding this wand, you gain a bonus to spell attack rolls determined by the rarity of the wand. Also, you ignore half of the cover when you make a spell attack.
There are multiple elements already incorporated in the game that give Focus more power than a traditional Focus would normally do.
A Warlock (once he has taken the Enhanced Covenant Weapon feature) and the Bard Swords College can use their melee weapons as a focus to cast spells.
Clerics and paladins can use their shields as spell casters.
All of these elements can be magical in nature and potentially give advantages to your spell casts and / or attack / damage rolls.
While each class may have some kind of empowered focus or weapon / element that can be used as an approach, not everyone will grant spells, but many can. If you are looking for something more specific, I would recommend reading carefully the elements that are currently manufactured within 5E through D & D Beyond, or the main rule books that are available to you.