Yesterday, on the evening of Memorial Day, I roped a few friends into Roll20 to do some live-fire testing of my homebrewed initiative rules (you can download them on the sidebar, or get them directly here). Note that this linked version includes changes made by virtue of this playtest, broken down below.
We focused primarily on Counting Initiative, which I was the most fond of, but feared the complexities it would introduce. Three combats of varying difficulty, player count, and monster count helped me give the system a shake down that was much better than my internal playtesting.
The playtest characters were all third level, and consisted of players from all levels of experience with 5th Edition and RPGs in general. At least one had never played 5th Edition before last night, but had plenty of experience with 1st and 2nd Edition as well as some other fantasy RPGs. Several were extremely experienced and I could rely on to break the system over their knee.
For clarity, the pregenerated characters I gave them were a human (variant) champion fighter, a mountain dwarf berserker barbarian, a half-elf lore bard, a human (variant) thief rogue, and a halfling (lightfoot, but reflavored as kender) hunter ranger.
The results were very promising.
As a quick TL;DR, Counting Initiative is designed to give players a little more tactical fidelity if they don't have to or want to take every conceivable action available to them. When rolling initiative, you roll a single d6 and add no modifiers (Dex-focused characters already get a whole lot, so while this does devalue Dex, I didn't think it was too awful). Operating under the six second round paradigm, the system assumes that a bonus action, a regular action, and a move counted for six ticks making up a round. Thus, if you took all three of those, your initiative count advanced by six.
However, if you didn't want to move, for example, you only took the Attack action while standing toe to toe with a monster. Thus, you only move up three ticks in the count. You can go again when the count reaches your current count plus three.
There's a handy-dandy chart. Since most DMs I know do their own turn tracking, this didn't cause players to do any more mathing than they usually did unless they wanted to optimize their actions.
Example: Aspira the Aspirant rolls for initiative and gets a 3. The DM rolls for the monsters, who get a 5. Aspira is first in the combat, and acts on count 3. She moves to close the distance (2), and uses the Attack action (3). Her count moves to 8, and when 8 happens, she goes again.
On count 5, the monsters act. Under Counting Initiative, all monster turns take six ticks regardless of what they do, to save DM headache. At the end of their turn, the monsters' next action is on count 11.
On count 8, Aspira goes again. She uses the Attack action to fell her current opponent (3), moves to the next in line (2), and uses Second Wind as a bonus action to heal some damage suffered from the monsters' attacks (1). Thus, her count advances to 14.
I was really worried this would introduce a lot of complications to the battles, but it ended up playing out very elegantly. The players correctly cited some intended effects; turreting or standing and swinging made you go more often, while movement and using extra actions slowed you down. Dual wielding weapons, by virtue of requiring a bonus action, helped provide some more incentive to wielding a two-handed weapon (less chances to crit, can suffer from whiffs a lot harder -- I know about Great Weapon Mastery, so give me a bit to get to that).
Readiness -- that is, the act of being able to use bonus actions and reactions -- ended up playing out quite smoothly as well, so players couldn't just spam 1 count bonus actions every tick of the combat. That is, you had to take any action before reactions and bonus actions were "ready" again -- even movement wasn't enough.
That said, it wasn't all wine and roses. The system has some bumps that need to be ironed out.
Firstly, ranged characters that were amply protected and did not need to move could be murderous turrets. Acting every three ticks can set up situations where you go twice before the monsters go again. When paired with feats like Sharpshooter and certain class abilities, archers could become extremely deadly. Melee didn't feel quite as scary here because they already tend to be at the most risk, being up in the face of monsters trying to claw their faces off.
Regarding Great Weapon Master fighters, my impetus for not rating them as high on the per-tick deadliness scale as a comparable Sharpshooter archer is due to needing to move to close to new targets. The archer can turret if not given pressure to relocate, whereas the frothing barbarian needs to wade through a sea of blood and steel to get to the next victim of the hewing greataxe.
Since the system already assumes that cantrips are 3 tick actions, and other spells are 4 tick actions, it felt logical to change it so melee attacks are 3 tick actions and ranged attacks are 4 tick actions. There's a bit of necessary aiming involved, so narratively it at least made some sense.
Second, monsters were too slow. Waiting 6 ticks to go again exacerbated some of the doubling, but I didn't want non-Legendary monsters to involve piles of disparate turn tracking when there were more than a couple monsters on the board. Thus, after a bit of throwing spaghetti at the wall, I think the comfortable magic number is 5. This would also be modified by certain creatures who are traditionally associated with being extremely quick (like some small fey, or kobolds) act every 4 ticks.
Third, small movements felt too punishing. The dreaded specter of the five foot step (or shifting, for 4th Edition alumni) reared its ugly head. Rather than provide a sliding scale of movement cost, it's probably simplest to have movement of only five feet cost 1 tick, while anything greater (including standing up from prone) to cost 2.
This also might lead to a slight buff of the Athlete feat, where by virtue of rules-as-written it would decrease the cost of standing up from prone to a 1 tick action (Athlete, among other things, allowing you to stand up from prone using only 5 feet of movement rather than half).
The playtest also did not adequately test spell durations and start/end of next turn effects. I would want round two of testing, with the above changes, to involve a bit of that for more rigorous review.
All in all, this was a huge success as far as the playtesting went. I am eager to see what comes next.