D&D 5E DM-Facing content to port into 5e from 3e/3.5+

[mellored]

A possibilitie 4e mechanics to bring over...

Any healing (at very least healing word) requires you to use a hit dice, getting the additional healing. Paladins can also use their own hit dice when they use lay on hands.
If you don't have a hit die to spend, you regain the HP but are still unconscious.

 

[Stormonu]

Stormwrack, too!

I knew there was another I was forgetting.

However, Dungeonscape (sadly) wasn't very impressive. I remember wishing it had been like the others environment books, but focused on spelunking, caves and the underdark - like the old 1E Dungeoneer's Survival Guide.

I will also say that @Micah Sweet is on the nose with 2E - if there is lore you want to carry over from previous editions, old 2E books can't be beat. In fact, a lot of 3E lore is abbreviated (and slightly revised) lore from 2E - for example a lot of the Lords of Madness info was drawn from 2E's I, Tyrant and the Illithiad books, shortened and touched up in places.

 

[Zardnaar]

Conceptually prestige classes, sone magic weapons, and classes come to mind.

Subsystems like the environment books, naval combat and things like crafting, panache or building things like Castles.

If you're looking at big rewrites some monster design and mechanics like SR can be tweaked.

Energy drains inflicting exhaustion vs level loss.

 

[Zardnaar]

I've drawn heavily on 3.5, 4E, Star Wars Saga and BG3 for my 5E rewrite.

 

[Lakesidefantasy]

Pathfinder's Gamemastery Guide is essential. And, there are so many other 3rd party things. Green Ronin's Advanced Gamemaster's Guide is great. They also have The Book of the Righteous which has fully fleshed out religions and whole systems of religions. Pathfinder's Ultimate Campaign is good; as well as their NPC Codex. The Codex is particularly awesome because creating NPCs is a horrible time sink in 3rd Edition. I would also recommend the Book of Vile Darkness (chef's kiss...but it's not for everybody). Finally, all the Ars Magica stuff. While not d20 System compatible, it matters little because most of it is lore anyway.

 

[JEB]

I recently got the 3.5E Magical Item Compendium.

2e's Encyclopedia Magica books are another great source of magic items for 5e games. (One of my players enjoyed the role-playing opportunities from their Ring of Fearlessness, for example.)

 

[jdrakeh]

I'd love to see an updated Ghostwalk.

 

[Li Shenron]

First and foremost, ability score damage as a special feature of selected monsters. I have done this sometimes and it works better and more easily in 5e than in 3e!

Somewhat related, but not easy to pull off properly, would be level drain. This definitely requires some thinking, I wouldn't probably bring it as-is into 5e.

An extended skill list inspired by 3ed would be interesting. I have already allowed players to make up some skill that isn't in the PHB list.

Prestige classes in their original concept, i.e. DM-designed and setting-specific alternate high-level progressions that could be granted as a reward and tied to the narrative, could definitely fit in 5e without conflicting with subclasses. This is not something I have tried, and would definitely require some work.

I also think that Weapons of Legacy and Book of Nine Swords could provide interesting additions.

 

[el-remmen]

2e's Encyclopedia Magica books are another great source of magic items for 5e games. (One of my players enjoyed the role-playing opportunities from their Ring of Fearlessness, for example.)

If I could get my hands on a cheap print version I would buy them without question

 

[Tonguez]

Monster Tempaltes would be something I'd bring over, maybe racial levels too

I agree the supplement books like Heroes of Horror were great, Book of Nine Swords etc