New editions for D&D and things that wish they were D&D continue to come out at a steady pace. 5e is a few years old, and new Pathfinder is gearing up for a second edition. One thing that frequently gets changed between editions is some tinkering with the combat system. Hit charts gave way to THAC0 gave way to BAB, saves went from five to three to six, HP is constantly inflating, and so on.
And it’s weird to me that none of the more recent edition changes have drawn any inspiration from XCOM: Enemy Unknown, because those guys nailed it. Obviously you can’t just do a wholesale conversion because there’s a lot of guns and cover and so on in XC:EU, but the basics are really strong.
The average XC:EU strike team is between four and six soldiers. There are four classes of soldier in the base game, a fifth added in an expansion, plus psi powers and (in the expansion) genemods that can serve for further customization. Each class has seven levels of progression with a new ability coming online at each level (no dead levels), and not only that, five out of seven levels have a choice between two. One option is often clearly superior to another in the base game, but mods have fixed that (and while that is not to the developers’ credit, the information is still out there to be used by people designing new games, so regardless of who gets the credit, there’s no excuse for new games not to be learning from this example). There is further customization available through equipment, with that bonus fifth class from the expansion getting completely unique weapons, armor, and secondary item options (with the secondary item being built into the armor instead of carried separately). Point of all this being, there are absolutely enough customization options for each character to be an individual PC. It’s not flawless as-is. There is no race option, which you’d want for a D&D style game because people want to be an elf and have that actually mean something, and you need more than just five classes (especially since the fifth class is only unlockable in the mid-game). As a proof of concept, though, XCOM units totally work as player characters.
Which brings us to the actual tactical combat system. Initiative is simple: First all the XCOM agents go. Then all the aliens (or, in some expansion battles, EXALT agents) take a turn. Each character gets two actions. The actions can be used to move, activate a special ability, use an item, attack, or enter into overwatch. Attacks and going into overwatch will end your turn early, even if it’s your first action. So will several items and special actions, so in practice this is a lot like having a move action followed by an attack action, but since both actions are interchangeable, you can have abilities that allow you to do things like use items without ending your turn, attack without ending your turn, and so on.
Overwatch also standardizes the concept of a readied attack, in that you attack but at a to-hit penalty the first creature that enters your line of sight. This lets you do things like have a hard to hit assault unit run out to trigger the enemy overwatch attacks so your more vulnerable support units can move in behind. In a D&D game, half-ish of your characters will be melee fighters who don’t get standard overwatch actions, but 1) half of them will also be archers and casters who use overwatch the same way XCOM agents do, and 2) melee characters could still ready an attack against the first person to get close to them or use a special action (which, like the sniper rifle attack from XCOM, can only be used as the first action) to ready a charge to move to and attack the first enemy who passes within range. Again, you will need to make adjustments, but you can definitely use concepts from XCOM to make a better D&D combat system.
Let’s also talk about numbers. One place where XCOM does not translate to D&D is that the computer can calculate an exact percentage to-hit chance using square-by-square distance and angles vs. cover in a way that would be unwise to translate to the tabletop. Having accuracy decrease noticeably within ranges that actually show up on the battlemat is a good idea (as is having something like the sniper, which is actually more accurate at longer ranges), but you want to limit that to something like “-2 penalty to attacks on targets more than 12 squares away,” not “-1 for every 3 squares” or whatever it is XCOM gets up to. Their cover mechanics pretty much work, though, in that cover seems to pretty reliably confer about a -20% hit chance, which comes out to a -4 penalty, which is perfectly reasonable.
But aside from to-hit numbers, XCOM’s got the numbers down in a way that’s easily translated to tabletop. First of all, health. Even on lower difficulty levels (which is what you’d want to emulate in terms of character durability), starting XCOM agents have only 6 health, while low level enemies have 3-4. Your first encounter with chryssalids is terrifying because they do a whole five damage and have eight health. This is enough to make a chryssalid crit a guaranteed insta-gib against early game units, and they show up right at the transition from early to mid-game, so if you’re even slightly behind the gear curve, you’ll have to fight them with early game units. Likewise, that eight whole health makes them take 2-3 hits to bring down with early game units, and they’re encountered in groups of 2-3. Even with a full team of six early game agents, clearing a chryssalid pod with no fatalities requires careful planning and often a callous exploitation of their tendency to split up to slaughter civilians, allowing you to pick them off one by one while their mates kill the innocent. This terror is inspired because they have eight health and do five damage. A lot of XCOM weapons have a damage roll that is something like a d3 or d4.
These low numbers make it easy to do the math in your head. You know, without checking a wiki or doing the math, that a chryssalid’s eight whole HP means that it’ll take two hits to kill (until you get plasma weapons and start laughing in their face while melting them in one hit). When a mechtoid rocks up with 20 HP, my first thought, before doing any exact math, was “Jesus Christ, that thing will take like four hits to kill!” (and this was with laser weaponry, which deal 1d3+4-ish damage). When I saw it could do 7 damage with a single hit and its attacks didn’t automatically end its turn, I had that half-panicked kill it kill it kill it kill it reaction, and you don’t get that if you have to pause and punch the numbers into your smart phone first. Obviously, whether or not you have to do that depends on how good you are at doing math in your head, but it is straightforwardly true that a system which gets the intended emotional reaction out of more people is better than one which gets it out of less.
XCOM’s cooldowns and ammo systems likewise rely on small numbers easily recreated at the table. Most weapons are out of ammo after three attacks. Most items are consumed after either one or three uses. Most special abilities that aren’t themselves consumables are on a refresh of between two and four turns. You could track the abilities with clever character sheet design by leaving a row of empty boxes somewhere on the sheet, maybe put a “ready for action” icon in them, with a place to write an ability name below. When you use an ability, place a six-sider with the appropriate face upwards, say, five for an ability on a five-turn refresh. At the end of your turn, decrease each ability die by one. If it’s showing a one, you remove it completely. The abilities that are currently available are the ones that do not have their box covered by a die (and are thus displaying that “ready” icon). Even end game XCOM characters only have four or five abilities, so buy a single eight pack of six siders for like five dollars and you are covered for the entire game and then some.
With ammo counts as low as three, you can represent them with poker chits, spare change, or more dice. Like, every time you take a shot, toss a die in the pool. When you reload, fill back up to three. Reload mechanics are going to be much more rare in D&D than in XCOM, but by tracking ammo abstractly in attacks rather than shot-by-shot, you can do it if you want to. Unlike with schemes to make shot-by-shot or blood point accounting or whatever easier using physical tokens, you don’t run into logistics problems where the average gamer doesn’t have enough random pocket change or spare dice to meet demand. You don’t need ten or twenty per player right off the bat, you usually only need one, and then a handful more come in piecemeal. Likewise, Roll20’s inability to store double digit numbers over its token icons is not going to thwart these systems the way it does any effort of making arrow-by-arrow accounting tolerable (it’s also worth noting that in XCOM, when you’re out of ammo, you aren’t useless for the rest of the mission, you just have to spend a turn reloading).
XCOM is full of good ideas for turn-based, squad size combat, and an alarmingly large number of them are somewhere between trivial (action economy) and only slightly difficult (cooldown abilities) to implement in tabletop. And yet, these good ideas go completely unused, even six years after that game released to an 89% metacritic score and enough financial success to spawn a (significantly less exceptional) sequel.