I’ve made many game variations, most of them Non-Gmod or source. I’ve had a load of success with certain parts of these games.
If you’re looking for popularity, the list is simple.
One- Equilibrium. Make sure you dont have any blatantly easy way to win a game, or a incredibly hard one. So, it’s a rather good idea to weight your guns to all have ups AND downs. let the players decide what gun they like.
Two - Personalization. Players are more likely to stick around and play when they can make the game their own. A good idea for doing this is Weaponry Attachments and such. It’d be even better if you made a dynamic armor system that sticks props onto players and stuff like that. Players love to make themselves look great and work great, so hats and stuff are always a good idea.
Three - Replayability.
A game gets boring when it’s the same over and over again, Unless it’s ungodly good or somewhere in between. Even then, it will turn into a clique game (Zombie Survival, Counter Strike, Zombie Master etc) So make sure that there’s alot of dynamic gameplay.
Four - Sensory Detail.
This is a very specific one i’ve found, you wont hear it as often. The only way i can explain this is the whole feel of the game and the tools and actions of the game. For instance, when a player picks up a sniper rifle, they expect it to make a big BOOM when firing and completely tear off someones head. make sure they see and experience those kinds of things. Make sure effects are good, Guns are satisfying to fire and have fitting noises, people are worth it and satisfying to take down, etc.
Five - Control.
Players are more and more used to certain styles of gameplay nowadays. If you’re making an FPS team shooter, Make sure that the controls are more or less the same and nothing is super different. it’s good to be original, but it’s not good to confuse the player.
Six - Reward
Players nowadays are seeking instant gratification, and landing a kill is less and less fulfilling for them. It’s easy to fix that. Things like levels and progression make this very easy. COD for instance is popular due to the ability to level up and gain new stuff. They even make an obvious, animated, sound effected symbol on your screen to show you that you did something good and was rewarded with XP right then and there.
I hope this helps, and good luck!