Is it possible for an arcane focus to boost spell attack or damage rolls? To act as a +1/+2/etc. modifier, like stronger physical weapons.
Absolutely it can!
A wonderful example of this is the Wand of the War Mage, (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 wand’s rarity. In addition, you ignore half cover when making a spell attack.
There are multiple items already built into the game that give Focus’s more power than a traditional Focus normally would.
A Warlock (once having taken the Improved Pact Weapon feature) and College of Swords Bard can both use their Melee weapons as focuses for casting spells.
Clerics and Paladins can both use their shields as spellcasting focuses.
All of these items can be magical in nature and potentially give pluses to your spell casting and/or attack/damage rolls.
While every class can have some form of empowered focus or weapon/item that may be used as a focus, not all will empower spells but many can. If there’s something more specific you’re looking for I’d recommend perusing the items currently made within 5E through D&D Beyond, or the core rule books that you have at your disposal.