Simple Option: Group Initiative
Use the average party initiative to determine whether the PC or monster turn happens first each round. An interesting effect of this is that
specifically targeting monsters that are likely to have high initiative becomes a valid tactic, since it can change which side has the highest average initiative. This may cause the PCs to get two turns in a row, if they previously had the lower initiative and now have the higher.
The disadvantages here are that having a high initiative benefits the party as a whole, but it doesn't do anything to differentiate fast characters from the others. There's also the unfortunate side-effect that taking out a monster with a low initiative will raise the monsters' average initiative and may end up giving them two turns in a row.
Alternately, you can just work out the group initiative scores at the beginning of the fight and never change them, which removes the possibility of either team getting two turns in a row.
More Complicated Option: Quasi-Simultaneous Turns
Each of the PCs declare where they're moving, what they're casting, who they're attacking, etc. whenever they post. Then once everyone's declared this, you resolve the actions in order of initiative. In some cases, an action might be invalidated by the results of a previous one: a target might be killed by a previous attack, for instance. The higher you are on the initiative list, the more likely your attack won't be wasted. We're still doing all the PCs, then all the monsters. This way, you avoid situations like the following:
- Fighter moves next to goblin and attacks
- Goblin moves away
- Goblin wins initiative, and moves out of range of the attack
But you might still have situations like this:
- Fighter moves next to goblin and attacks
- Rogue moves next to goblin and attacks
- Rogue wins initiative and kills goblin
- Fighter no longer has a target.
To cut down on situations like that, you might allow action declarations like "I move next to the goblin and attack. If the goblin dies before then, I attack the orc next to it instead." The movement is the same in either case, so the other PCs have a good idea of what you're doing and can plan their moves accordingly (maybe allow it to vary by a 5' step or so, if that's too restrictive), but you have some flexibility.
A disadvantage of this method is that while having a higher initiative than the other PCs makes a difference, having a higher initiative than the monsters doesn't really matter. Although if you aren't using a battlemap, then depending on how precisely you're tracking everyone's positions, it might be safe to resolve the PC and monster turns simultaneously without running into problems like the "goblin moves away from the attack" example, in which case this disadvantage disappears.
Let the PCs declare something like a readied action, but using their next turn instead of the current one. Unlike a normal readied action, an opposed initiative check is required to determine whether the action goes off in time. For example:
- Player Turn 1
- Fighter: I attack the goblin. 16 to hit, 4 damage. Also, if I see the wizard casting, I'll try to attack him with my mace. I have +4 to hit, d8+1 damage on that attack.
- Thief: I sneak behind the orc. 18 on the hide check.
- Monster Turn 1
- Wizard attempts to cast, and is close enough that the fighter can reach him in one turn. DM rolls an opposed initiative check between the fighter and wizard to see if the interrupt happens. The fighter wins, so the DM rolls the attack as specified and determines if the spell fizzles.
- Orc rolls spot vs the thief and fails, then shoots the fighter with a crossbow for 5 damage.
- Player Turn 2
- Thief backstabs the orc.
- Fighter skips this turn; she used her action interrupting the wizard. If the wizard hadn't attempted a spell (or the fighter had lost the opposed check), the fighter would take her turn normally.
Actual readied actions work as normal; you need to spend your current action to do it, and that action is gone whether or not the trigger happens, but there is no opposed check necessary.
If the interrupts prove too powerful, you can dial back their reliability by giving a bonus on the opposed roll to whoever's turn it is.
The Interrupts option can be combined with either of the others, or used by itself.