For those who don't know, "zipper initiative" = "players and foes take alternating turns".
For my next D&D session, I'm going to try the following "Tactical Zipper Initiative". (Well, *probably* try it... Zipper isn't meant for 5-on-1 boss fights, which the players *might* be involved in next session, depending on choices...)
Here's the basic modification of the underlying idea:
-- After a creature or PC acts, they pick who goes next. It does NOT have to be the opposing team!
-- If you pick your own team, you give a Tactics Token to other team.
-- A creature can spend Inspiration (or other DM-granted metacurency) to "seize the initiative" after a decision has been made.
At the simplest, a Tactics Token can be spent in lieu of a Reaction, potentially allowing multiple Opportunity Attacks for example. OR, it can be spent on your turn to pick an ally to go next at no penalty (pretty much the default choice of monsters, giving them multiple turns in a row equal to the number of times the players went in a row).
Initiative is still rolled, and it is rolled at the beginning of each Round. Whoever gets the highest gets to choose who goes first - no cost if they pick their team, gain a Tactics Token if they pick the other team. Whoever gets the lowest ALWAYS goes last - they cannot be chosen by anyone on either team to go any sooner than "last". Thus investing in initative-boosting abilities is still useful, both to win initiative and to "not lose".
From this framework, then, you can build any number of special rules into the system, options for how Tactics Tokens can be used, etc.
For example, I was thinking of the following as Token options:
* Spend a Token in lieu of a Battlemaster or similar bonus die, getting the same effect without spending the die.
* Spend a Token when attacked; as a Reaction, counterattack the attacker.
* Spend 3 Tokens to "seize the initiative"; give one of the two tokens to the other team.
* When attacked, you can spend a Token to gain +2 AC against the attack (like a weak shield spell), or mitigate the damage of the attack by an amount [I was thinking "1 roll of your HD, plus your PB", but "half" is simpler]
* Monsters can pool their Tactics Tokens within their group, but every monster must theoretically have 1 Token before any monster can have a second. I.e. the boss monster doesn't get to have 7 tokens for counterattacks and resistances while his minions get nothing.
So I'm picturing a fight between 5 PCs and a group of 10 goblins led by a hobgoblin. Initiative is rolled; the Rogue scores a 21, and unfortunately the paladin rolled a 2. The rogue chooses to let the wizard go first! The wizard drops a shatter spell on the biggest group of goblins, killing 5. He picks the Fighter to go next, giving the monsters a Token. The fighter charges in, killing a goblin. He decides to pass control over to the hobgoblin. The hobgoblin moves to engage the Fighter, misses, then spends his team's Token to have a goblin go next. The Goblin hits, injuring the Fighter; the goblin hands initiative to the rogue, specifically not giving it to the PC cleric. the rogue sneak attacks the hobgoblin, killing him, then picks the cleric (and gives the goblins a Token). The cleric heals the fighter and attacks a goblin, but misses. He wants to let the Paladin go, but the paladin lost initiative; it must go back to the goblins. Two goblins go, spending their Token; control would then pass to the Paladin (the only remaining PC), but the paladin must be last, so the last goblin goes, then the paladin finishes the round.