I think you might be missing the point @iserith is going for here. He’s not trying to use house rules to make 5e play like 3e, he’s trying to run a 5e game that has lots of little references to 3e.The biggest issue I see with a lot of the suggestions here is that while they are all bits and bobs that have roots in 3E... most of these things will never actually show up in your game. And even if they do, they will be no different than what you could already see in 5E and thus I don't seem them really feeling like there's a difference.
Chaotic barbarians and Lawful monks? Sure, you can force the issue during character creation. But have you actually had so many 5E games with Lawful barbarians and Chaotic monks that this change would actually feel different than what you've already played with? And this even suggests you end up with a barbarian and/or monk being played in the first place.
Removing stat caps? It's going to be such a long time down the campaign road before that ever becomes an issue that that's not going to be any rule that will make your current game feel 3Eish.
Axiomatic weapons? How often do you put in magic weapons into your games where a bonus to Chaotic creatures would actually change the feeling of the game? Isn't that just like any other 5E magic weapon except it gives a couple bonus damage dice to a different selection of monsters than the standard dragons or undead?
Monsters that have certain abilities like they had in 3E? Those are all well and good, but that requires you to actually put one of those special monsters into your campaign, and it requires you and your players to actually remember that X special ability was normally only found in 3E and not in the 5E version. An iron golem immune to magic? I mean if you've experienced enough iron golems in 5E that haven't had it to make you actually feel like magic immunity has been missing this entire time, then okay. But I know I couldn't tell you what abilities the 3E and 5E iron golems had differently (or almost any monster for that matter) to make putting a 3E one into a 5E game would suddenly make the game feel like 3E.
Basically what I'm saying is that unless you've played 3E and 5E so much to the point that not having these 3Eish things in the game has actually made you and your players stop and go 'Huh... remember when..." then putting any of this stuff into your 5E game I suspect will just feel like a 5E game with new bits in it. Because it's none of these little things that make 3E and 5E different... it's the entire ruleset-- character creation on up.
For 5E to feel more like 3E at the barest minimum at character creation... you'd need to strip out probably 50% of all class features and turn then into Feats, while also taking the current Feats and cutting all their abilities in half to create two smaller ones. And then take every other class feature and de-power it, while making a Feat that becomes the standard version as 5E stands now. Then give everyone more feats with which to "customize" their character, so they build their PC up like they did in 3E. But then again... even something like that probably won't accomplish anything, because that'll just have players re-creating their 5E character archetypes they have now, but it being built piecemeal via Feats, rather than just have their power from the get-go.
I don't mean to be a Debbie Downer about this... and quite frankly I might very be the absolutely wrong person to even bother talking (given my general disdain for game mechanics to begin with.) But I know for me... the only way to get a game to feel like an older game is to actually play the older game. Because it's not just isolated bits and bobs that make a game what it is that you can just "patch in" to another one... it's the entire essence of the game built from the ground up. So no matter what I could do... I would never be able to recreate the feeling of a 4E game using 5E, even by including all the variant tactical combat units into it. Likewise, there has never been a 3E, 4E, or 5E game that has even come close to being like AD&D. Even when I look at the 5E Basic Rules, that doesn't say "AD&D" or "2E" to me... instead it says "A 5E game with a specific character pool that any of us could be playing with right now."
Now if you can actually demarcate these rules bits in your own mind where you can achieve the 3E enlightenment using the 5E engine... then I congratulate you and say go for it! I just know I am incapable of switching an emotional feeling of a game via intellectual mechanical changes. All the best!
Not really the question, but I’ll point out that I never stopped DMing 3.5e. As DM, what edition you run is your call. There’s no shortage of players who like 3.5e, if your players are new to D&D or have been playing for a long time - I have both in my games.What else would you play up to make a D&D 5e game reference D&D 3.Xe in small ways?
Giant animals are dire animals with bone spike projections.What else would you play up to make a D&D 5e game reference D&D 3.Xe in small ways?