So for background; in my world for an upcoming game magic is a new and toxic thing to the world, mutating and giving powers to all the creatures and the terrain itself. The magic weapons in the world reflect this idea in that they will have some sort of drawback. 

My first idea was to create my own wild magic type inspired table that would activate every now and again, but im not 100% in if that's what I want to do. I was also thinking instead of doing it to where each item would have a specific debuff to counter the buff. For example a flametounge sword or a similarly fire damage dealing sword would make its weilder more susceptible to fire damage. (+1,2, and 3 weapons and armor arent considered magical in this and are instead considered finely crafted equipment and such wont suffer drawbacks). So that leads to why I'm posting, I was wondering if I could hear some ideas on side effects that I could use or just to inspire me, along with maybe your opinions on this.

Thanks <3

-Manna 

Because so many magic weapons in D&D have a +1(or better) property in addition to their specific buff,  and so many creatures are resistant to non-magical physical damage, it may be easier and more consistent in your world to say +1s are magical, but have a unique, lower cost. Unless you’re going to remove those creatures from their enemies list, or move right past Uncommon rarities in your treasure drops.  Which I suppose you could justify if Rare and Legendary weapons all have a drawback that makes players more vulnerable as they become more powerful.  But for +1 magic weapons, maybe it costs the wielder 1 hp every time it deals damage. Or maybe when it deals max damage, you have to roll a damage die against yourself, as it was using your vitality to fuel its magical bonus.  Maybe those are two different varieties of weapon debuffs, and you roll on a table with 4 other options like those when you find the +1 weapon to find out which cost is associated with this weapon’s magic infusion. Other magic drawbacks could be

• Animals and beasts are instantly hostile to anyone carrying the weapon.
• (For +1 Armor, shields) if an attack gets through the armor, roll 1d?, on a 1 you become frightened of the attacker. Maybe that’s a standard curse for +? Armor and shields. +1 armor is a 1d8 chance, +2 is a 1d6 chance, and and +3 is a 1d4 chance. Or the other way round.
• It’s a +1 weapon, but you lack your own skill while holding it.  You lose (can’t count your Proficiency bonus for) proficiency in Athletics and/or Acrobatics while making checks for those skills with this weapon in hand.   
• The same thing as above, but with INT based skills.
• The grip of the sword is toxic. A CON save is required each time you draw it, or you are poisoned for 1 hr.. The DC maybe starts at 5 and gets higher over prolonged ownership as the toxicity builds up in your bloodstream.  Takes a week without touching the blade to cleanse your blood.
• For a named rarer weapon (like a mace of Turning, or smiting or something cleric-like) it makes you resistant to healing. Spells and potions heal half the rolled value. And short rest hit die as well. Long rests should still heal to full, I think.  Though, I toyed with the idea that a long rest can only heal half your max HP at a time. 
• The weapon’s magic is inherently merciful, and it won’t allow itself to be used for opportunity attacks.
• The weapon glows with a faerie fire that extends to its wielder. You can’t become invisible while carrying it. (I’d say ‘and attacks on you while holing it have advantage,’ but that seems too much. Maybe attacks made against you in lightly obscured, or heavily obscured environments like fog, are no longer at disadvantage.)

These are all lovely ideas! I'll be sure to refer to this when making an item and brainstorming for items. You're contribution and help is much appreciated! 

-Manna <3

I love items with unpredictable magic. For example:

• I gave my Paladin a cursed shield. If an attack misses the Paladin and he uses his shield to block it (defined as missing his AC by 1) then it casts Vicious Mockery at the attacker. If an attack hits him that he should've been skillful enough to block with his shield (defined as exactly matching his AC) then the shield casts Vicious Mockery at him instead. 
• Boots Made for Walking (found in homebrew). They can double your movement for a minute, but they really want you to use your whole movement, which I interpret as forcing the wearer to make a strength saving throw every round that they don't use their full movement and walking them in a random direction if they fail.  Also if you don't take them for a walk every day they might walk away (possibly with you in them while you're sleeping).