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