I have been discussing this off and on for a while in casual, unstructured settings (like in person, where there is no fear anyone will write anything down or remember it later), but it would serve me well to write it down. A few months back, I spent some time looking at the retiarius. These guys were super cool, but they are near impossible to make work in D&D 5e. The main reason is that the spear and net just aren’t very good weapons.
The spear is both simple and versatile, but the damage is 1d6 if used one-handed, and 1d8 if used two-handed. Sure, it can be thrown, but the range is 20/60. The trident is exactly the same as the spear, but is a martial weapon, and is five times the cost. The only advantage it has over weapons like the battleaxe and longsword is the ability to be thrown. This is cool in a pinch, but it’s not like anyone is going to reasonably be carrying around a mass of spears and tridents to hurl at enemies (returning enchanted weapons not withstanding). If someone wants to throw things, javelins and handaxes are there for you (same damage, lower weight and cost, and javelins have better range).
The net has its own problems. The net is seemingly powerful, due to the fact is applies the restrained condition. However, it is extremely limited. The net is a ranged weapon that has a range of 5/15. This means you have disadvantage on the attack if you are further than 5 feet away from the target, but you also have disadvantage on the attack if you make it within 5 feet of the target, because it’s a ranged attack made within melee range. If you do land the attack, it only works on Large or smaller creatures, doesn’t work on formless creatures (like an ooze), can be broken out of with a DC 10 Strength check made by the creature caught in the net (or an adjacent ally), and falls apart if it takes 5 or more points of slashing damage. Oh, and no matter how many attacks you have, you can’t make any other attacks if you use the net. Don’t get me wrong restraining a creature is a big deal, but this is so bad as to not really ever be used, and bad guys using it would end up having a rough day.
You could certainly take Crossbow Expert and Sharpshooter to overcome the limitations of net. In my opinion, that feels bad. You end up with two feats to overcome the deficiencies of one weapon. As for spear and trident? Well, there isn’t much help there, at least not until the Unearthed Arcana feats become canon. So what is there to be done, besides abandoning the concept completely? Luckily, weapons in 5e already have a system in place to lend a hand, and it’s name is weapon properties.
This isn’t anything new, and dates back to at least 3e (probably further, but I’m not really in a place where I can dig into things like that at the moment). 5e comes close to being a full weapon template system but seems to have back off at the last moment. Take the aforementioned trident. It has the exact same stats as spear, but costs more, weighs more, and is a martial weapon. There isn’t any advantage to using a spear over a trident, and all the martial designation does is further restrict who could use it. The same is true for the scimitar and shortsword, and the glaive and halberd. These are more or less redundant entries just to allow exceptions and specificity within class design. The monk class goes the opposite way entire and encourages you to describe your weapon choice in any way you want, in order to make a cool weapon a monk might use. The example used is describing a club as nunchaku. This is directly at odds with the weapon system as published. The monk could describe a shortsword as a scimitar, but that wouldn’t make sense, since scimitar is a distinctly different weapon in the chart. See the disconnect?
The existing weapon properties are ammunition, finesse, heavy, light, loading, range, reach, special (lance and net), thrown, two-handed, and versatile. I am not advocating a return to a gigantic list of weapon properties, but a small expansion could certainly do wonders for some of these seemingly redundant, under utilized weapons. The first property I am suggesting is maneuverable.
Maneuverable. If a weapon has the maneuverable property, any saving throw made against a maneuver performed with this weapon is made with disadvantage. If making an attack with a maneuver is the only attack you make this turn, the attack is made with advantage.
This might not be as elegant as it could be, since it carries a little bit of if/then logic with it. Hopefully the burden of knowledge isn’t too high. I believe it should be simple enough to not cause too much issue, or slow gameplay down in any meaningful way. I also don’t think this is out of line, power wise. Causing disadvantage on a handful of maneuvers isn’t as big of a deal as say, getting a flat +10 damage bonus on every hit. It also addresses a shortcoming with net, essentially making the attack neutral if you possess the ability to use maneuvers. The only potential issue I see off-hand is that it gives battlemaster fighters even more options. Even then, it’s more of a focus on utility than raw power.
For characters who are not battlemaster fighters, they would need to pick up the Martial Adept feat in order to benefit from this weapon property. This is still better than needing to pick up two feats in order to make a non-standard weapon even usable. The biggest issue is that I would want to give this to both spear and trident, unless I changed one or the other. Frankly, I’d be fine with giving spear the finesse property. This is just a result of watching way too many martial arts movies, but whatever. It’s not like it would hurt anything to allow dex characters to wield spears, either one or two-handed.