Magic Weapon difference from 3.5 to 5E

[Willpower]

So, I am both running and in a 5E game, though fairly new at the 5th Edition as is the GM of the game I play in. The difference is I am more rules oriented than most of the other players. Not truly a Rules Lawyer, but I understand them and its best to break rules if you understand them first so things don't go CRAZY.

Anyway, in the game I am playing in, I play a Bear Totem Barbarian (They SOO ROCK). The GM recently decided instead of taking his campaign to 20, he is going to end it at 10, but doesn't want to diminish his main bad guys. (He wants to end it early, so the characters can be used in other campaigns still) To do this he wanted to give everyone in the group special magic items that will help them deal with the aforementioned bad guys. Mine was the first to receive his weapon. Since he wasn't as big in the rules as I am he stated that what I received was a +5 Maul with an additional +5 elemental effect (Any Element I choose that can change when I choose) whenever I am in contact with the ground. He wanted me to work out the details. Though looking through the magic items in the DMG, it appears magic weapons don't go to +5 anymore, the most I have found is +3. So, I am wondering what are the basic differences in the way magic items are done anymore. He wanted me to not only design my weapon, but another players as well. I have a grasp on that and will detail the beginnings of that weapon on the next post.

[Willpower]

Just spent a half hour describing the weapon I am working on, and when I hit submit the stupid webpage wanted me to log in again. So FUDGE it, you don't get to hear about the weapon now. I may take the time to put it in later tonight when I have time, but right now this website just pissed me off. I was already logged in and it erased all my work. It was autosaving and everything, and now its all gone.

[FilthyLucre]

So, I am both running and in a 5E game, though fairly new at the 5th Edition as is the GM of the game I play in. The difference is I am more rules oriented than most of the other players. Not truly a Rules Lawyer, but I understand them and its best to break rules if you understand them first so things don't go CRAZY.

Anyway, in the game I am playing in, I play a Bear Totem Barbarian (They SOO ROCK). The GM recently decided instead of taking his campaign to 20, he is going to end it at 10, but doesn't want to diminish his main bad guys. (He wants to end it early, so the characters can be used in other campaigns still) To do this he wanted to give everyone in the group special magic items that will help them deal with the aforementioned bad guys. Mine was the first to receive his weapon. Since he wasn't as big in the rules as I am he stated that what I received was a +5 Maul with an additional +5 elemental effect (Any Element I choose that can change when I choose) whenever I am in contact with the ground. He wanted me to work out the details. Though looking through the magic items in the DMG, it appears magic weapons don't go to +5 anymore, the most I have found is +3. So, I am wondering what are the basic differences in the way magic items are done anymore. He wanted me to not only design my weapon, but another players as well. I have a grasp on that and will detail the beginnings of that weapon on the next post.

There are no guidelines or formulae like there were in the glory days of 3.5. I would suggest however that you conform as best you can to the limitations you find in the DMG. You correctly noticed that there are no weapons that have a bonus higher than +3. However, there are weapons with a +3 bonus that have additional effects.

[Keravath]

One of the main differences between 5e and 3.5e is the concept of bounded accuracy. There aren't that many "+"s to anything and those that exist are limited. For example, without magic items, the to hit ranges from about +5 at level 1 to +11 at level 20 (assuming 16 and 20 attack stats). AC also runs a similar or even smaller range.

This is why all the magic weapons and armor in 5e are limited to a maximum of +3 in the published materials. There are some cases where you can get more (for example a level 20 barbarian gets up to 24 strength, or a character with a girdle of giant strenth) but the range is otherwise pretty limited. (Even legendary items are only +3, eg holy avenger)

Bounded accuracy means that even a level 1 character with a +5 will hit a high level character with 22 armor class (+1 plate and +1 shield for example). Even low level threats can remain relevant compared to previous editions.

So unless your DM has home-brewed an off the wall monster, your level 10s should still be reasonably effective against it since you should have around +9 to hit without magical bonus on weapons depending on ASI choices. 5e also has fewer save or die effects so the chances of your party remaining effective against an opponent lasts a bit longer than previous editions unless you are unlucky. Anyway, cool magic items can be fun but you might want to chat with the DM about the monster involved since you may not need a +5 maul which will boost your to hit closer to a level 20 character with a +3.

[Kane0]

You can have a magical +0 weapon that has the special effects you want as well, the + numbers are lower because the value range is much narrower in 5e (they call it bounded accuracy, basically the d20 roll should always be able to influence the result).

If he's handing you a +5/+5 weapon in 3.5 terms that's basically a maxxed out item for the edition on par with artifacts, which would roughly correspond to the legendary rarity for 5e. That's a +3 item (no other bonuses) or something like a vorpal sword or Staff of the Magi.

Flavor abilities are always more fun and interesting that straight +x to hit and damage, so if you can provide some insight into what the weapon(s) should be doing and their flavor that will go a long way to us facing you in the right direction.

[J-H]

I suggest looking through the legendary-level items in the DMG. Not even an artifact-grade item gives +5.

A +3 Maul where you roll 1d4 for an on-hit elemental effect would be plenty powerful... ie:
"Maul of the 4 Elements": This +3 Maul draws from the power of the classical elements. On hit, roll 1d4 to determine what happens:
1 Earth: The ground shakes under the target, and it must make a DC 14 DEX save or be knocked prone
2 Air: The target must make a DC 14 CON save or be sickened as the breath is sucked out of its lungs
3 Fire: The target takes 2d6 extra fire damage
4 Water: The target takes 2d6 extra poison damage

Remember, saves do NOT scale in 5e. A 20th level Wizard has a >50% chance of failing the DEX & CON saves this weapon hands out. Generally, you're going to do extra damage half the time, put out a very good de-buff a quarter of the time, and have enemies make their saves about a quarter of the time.

Prone enemies give you and others advantage on hitting them; sickened enemies have disadvantage on most of their rolls.

[Contrast]

The GM recently decided instead of taking his campaign to 20, he is going to end it at 10, but doesn't want to diminish his main bad guys. (He wants to end it early, so the characters can be used in other campaigns still) To do this he wanted to give everyone in the group special magic items that will help them deal with the aforementioned bad guys.

This train of thought concerns me a bit.

A +5 weapon is insanely good in 5E but it nowhere near makes up the difference between a level 10 and level 20 character. Apart from anything else the level 20 character is going to have twice as many hit points and be able to do things like attack 8 times in a round or cast Meteor Storm for enough damage to instantly KO almost all level 10 characters from full health.

Also if the intent on not levelling these characters up is so you can use them in a future campaign, do you really want to give them brokenly powerful items which will then either totally skew the next campaign or need to be yoinked back out of the players hands?

There is also a bit of a problem if you're designing these items to help in a boss fight you (presumably) know little about. Robe of the Archmagi is really powerful but might not help much against a boss who isn't a spellcaster who has legendary resistances out of the wazoo.

If you really want the power of level 20 without the time to get there and ability to shift to another campaign with the same characters after, why not have the magic macguffin reward be some blessing which temporarily ages your characters to their prime at the critical moment (ta da you're level 20 for the boss fight) which wears off after the fight?