Spawning points were one of my more popular 4e house rules.
4e gets a lot of flak in the RPG community. I don’t expect to change anyone’s mind and convince you that it’s the greatest game ever. I do think there are some ideas that were improvements on 3e or solidified rough 3e ideas which are core to 5e or other modern RPGs. Things like standardizing the language about resource recharging for example. 3e, you’ve got at will, and per day, and towards the end you start getting the awkward “end of the encounter” language which was awkwardly metagamey. Then 4e standardized on at will, encounter, and daily powers. It’s a good framework, and 5e made it less metagamey with the short and long rest mechanic. Alternatively, the idea that monsters should have roles, which work like classes for monsters. The execution sucked, but the idea’s right on target. A mind flayer has a lot more in common with a lich than an otyugh, and yet the flayer and otyugh are both aberrations. Why not give the flayer and lich a similar powerset, and the otyugh can share some traits with a zombie or something? Without rambling forever, I’ll wrap with the idea that 4e is worth a look, even if it’s not a game I’d recommend any particular group play.
On topic, I ran 4e from 2007 to 2010 as pretty much my exclusive fantasy RPG. I’d dabble in non-fantasy RPGs, but like 80% of my RPG were 4e. 4e needed a ton of house rules to get running and really needed a lot of DM work to tailor it to your group, with things like some level of math fixes, monster HP reduction, or going so far as to debate which of the reams errata to apply or not. The easiest one we implemented was also the most popular, as these things tend to be, and it involved minions. For the unfamiliar, 4e introduced an idea borrowed from Feng Shui’s mooks, minions. These were monsters (never players) that had offense and defense appropriate to a monster of their level, but never had more than 1HP.
The really fun part of this is that you can fill the battlefield with minions, and give your players the option to just mow down enemies. By XP Calculations, 4 minions are the same as any other monster, and you can roughly convert normal monsters into minions by dropping HP to 1. So when doing encounter design, you can quickly go from a boss, two soldiers, two regular enemies (Party vs 5 foes, balanced 50/50) into a boss, two soldiers, and 8 minions. A battlefield with 10+ enemies is a lot of fun. In 3e, you throw 12 minis on the map, and you know you’re probably in for 3-4 hours of combat, unless your party goes heavy with save or die spells. Frankly, same for 5e. Minions let you do this. 4e combat can be a slog for other reasons, but hopefully your other house rules are also fixing that. There were boat loads of them, and they were very common. The downside to minions? Any damage whatsoever dropped them. This was fine most of the time, but sometimes you’d have stuff where a PC has an aura of fire surrounding them dealing an automatic 5 damage per round to anyone near them. Suddenly someone is immune to minions. Or they all go down anti-climactically to a single fireball. In practice, folks very often realized that the 1-4 XP budget for minions wasn’t actually enough. Often times people would say minions are no XP, or 1-8 or something. The Spawning Points house rule was fairly simple, and IMO made them very dynamic.
Spawning points are shamelessly stolen from Gauntlet Legends, which stole so many quarters from me back in the day. So if you’re unclear on how it works, keep that image in mind.
For any encounter where you plan to use minions, you should include a spawn point. This spawn point produces 1d4, or sometimes 1d6 minions with a recharge of 5 or 6. When you roll initiative, it spawns some right away. Then, you roll the die, and at the top of the initiative order, it spawns again. Minions spawn next to it, as best they can, and depending on the sort of spawner, you will have to be flexible. All spawners should be able to be destroyed or stopped by an obvious mechanism that usually takes a round to shut down by one player, with the exception of the boss spawner. I often, but not always, include a max number of minions each spawner can support as well. If it produces 1d4, I cap it at 4 at any one time, 1d6 capped at 6, and so on.
Base Elemental Spawner: A standing brazier filled with hot coals, cold water, pure earth, or smoke which act as a conduit to the elemental plane of fire, water, earth, or air. Every spawn, it spits out 1d4 elemental minions.
Support Troop Spawner: Someone shouted for reinforcements. More show up every couple of rounds through a doorway. If you can bar it, or otherwise reinforce it, they will stop showing up. Every spawn 1d6 troops show up.
Graveyard Spawner: A standing mausoleum curses the graveyard. Every spawn, 1d4 ghouls, zombies, skeletons, or whatever under make sense, burst forth from the heavy stone doors. Block the entrance or they keep coming.
Dark Alter: This Dark Alter curses the ground around it. Every few rounds, 1d4 skeletons crawl up from the ground within 60ft of it. Smashing it ends the effect.
Spawning Pool: This fetid pool of brown liquid teems with crawling life. Every spawn, 1d6 minion aberrations crawl free. A dose of fire will ignite the surface material and end the spawning.
Boss Spawner: The boss spawner is the one exception. Unlike most spawners, this one isn’t easily shut down. The boss can use their minor action to summon minions. This could be skeletons crawling up from the earth to serve the lich, or it could be a guard captain yelling for help, or a wizard summoning astral constructs. The big difference here is that the spawner can’t be quickly smashed or dealt with. You have the lich raise a staff and summon skeletons or something to let everyone know they need to stop him, but in general spawns are happening every round of the encounter.