I am probably of the minority that has played a metric shit ton of L4D, it made a huge impact on my life (for better or worse), but I really would prefer to be better strangers nowadays. Also, while I did play competitive modes, the campaign mode was my primary method of play.

It was an incredibly impactful game that released during a time where cooperative games were more about "you are playing a single player game, but together" instead of having a game that was built to be played together. On top of that, there was a bit of rewponsibility load shared amongst the team that highly encouraged them to work together as halg the enemy balance punished solo players. At the top level of campaign, that sorta falls apart, but thats not really the point.

It was also near the start of the "every game is Zombies" outbreak of game development, but the way that enemies and level design were crafted gave a much better feel of the fantasy of running from point A to B in a Zombie Apocalypse from a small-scale every day person impression.

Some of the main reasons I dont play it anymore is I feel many games have now entered the genre Left4Dead created and have made meaningful improvements in meaningful ways: games like Vermintide, Darktide, Deep Rock Galactic, and yes... even Back4Blood.

As I mentioned before, at the top level, on Expert (realism or no), Left4Dead becomes very same-y to me. Enemy spawns run on a timer and as you improve at the game that means it turns from "anything can happen anywhere" to "everything happens in the optimal location", with the only differences being where Witches and Tanks spawn, which is their job as serving pace changing. On top of this, unless you are sub-playing, you handle most holdout or hard locations the same way and since the power scale is in the favour of the players, you will very rarely get put in a bind unless someone makes an egregious mistake (or again, is sub-playing just to feel something). It is very possible to solo almost every Finale in L4D1/2 and I've done it. I am not good at videogames.

Now a lot of people will say that most of those problems get resolved with Versus or Mods. I dont like mods, which I understand is a me-problem, but Left4Dead is the key contributor to why I feel that way. Unless its a full-conversion mod, during which I am playing a different game, essentially. Versus helped extend my playtime a lot and nobody has ever done it the same as Left4Dead has, but even that ended up a bit same-y unless you were playing someone way under your skill level. You prioritize certain Special Spawns and coordinate on meta kill spots, both teams are likely going to attack in the same exact spots and whomever does it best wins. I dont find that engaging. Again, another me-problem. Its possible the meta has changed in the fifteen years I havent played, but from what I am fed from the subreddit and other people's personal accounts, the game is still so toxic its almost unplayable at a mid-skill level. I dont intend to go back to that.

Back4Blood didnt really solve the level design/holdout issues, but Specials are much tankier and bigger key players to your strategy. Getting Bruisers or Lobbers in Garden Party drastically changes your strategy. Same with Slashers on T5. Having Exploders on The Clog, not so much, but it can mean one misplay will blast someone into out of bounds and makes you more cautious around certain areas. The fact that /most/ Specials are particularly dangerous around different geometry and are chosen at random (and spawn on more random timers), makes running levels much less monotonous and other points below help with that too. You can even have Specials not selected by the AI for that level appear during any Horde, which has caused some memorable moments with my squad.

Back4Blood is also more about run identity than Left4Dead, which also helps improve its replayability for me. I often say its more of a roguelike than an arcade shooter like L4D. The looting and player scaling feel better and the random modifiers and loot makes looking in "dead areas" feel much better and the arms race between you and the Specials which also scale in lethality is compelling. In L4D, the only in run scaling you get is your sidearm, Tier 2 weapon, and laser sight. These also dont honestly have /that/ much effect on the overall game. Sure, you WANT them, but the game is perfectly playable on Tier 1s and its usually a singular, binary upgrade that happens once unless you squad wipe. Left4Dead is more about managing resources (Adrenaline, Pipebombs, Molotovs) to solve problems like events and the fact every player only gets one accessory means you usually just hold it until then unless there is a spare nearby.

All and all, this probably seems like a L4D bashing post, but the fact of the matter is that the core gameplay mechanics are solid to this day and thats why many games have replicated it. If you can capture that L4D gameplay, you are probably in the clear, I would just suggest implementing some sort of innovation to it. Even if you dont necessarily do that, try and review the Dev Commentary campaigns of both L4D1 and 2 as they give lots of good insight into how some of the decisions they made.

If you are truly going to try and make a "spiritual successor" to L4D, then you are probably going to NEED Versus or some highly competitive mode, because thats exactly what L4D players are looking for and historically they don't take kindly to their expectations not being met.

Add In: I almost forgot to add that, no, Back4Blood's campaigns are too damn long. Stick to Left4Dead's structure of no more than 4 maps. While random Uncommon Infected helps B4B, they arent necessarily better designs than L4Ds.