My fave way to homebrew magic items :

• Take an existing magic weapon, usually a longsword
• Make it literally any other weapon suitable to the party (Mace, warhammer, halberd, dagger)
• ???
• Profit

Or I just remember some magic item or NPC from Baldur’s Gate / Icewind Dale (the video games) and roll with that. My favorite being handing out heads of the flail of ages, that’s always fun

There’s also me who basically redesigns the game for a super-specific fanfiction campain and only keeps the dice rolling and character sheet mechanics.