No matter the system, you now have a group you feel is unbalanced for your kind of game.
How can you deal with that ? There are multiple ways. I've personally used many different ways, throughout the years in "similar" situations. I sometimes had to change, or refuse to change, the nature of a campaign after a character's death but I also have had to do so for other reasons like a player simply wanting to change character or changes around the "table".
As a DM, what do you prefer ? What do you believe your players would prefer ???
You basically have three choices.
Do not get me wrong, you have way more than 3 choices. But you have two "easier" choices, which in my experience make for better/easier story telling.
- Adjust the campaign to your group
- Adjust the group to your campaign
- Status Quo
In the past, I used all 3 solutions in similar circumstances.
About adjusting the campaign to your group or adjusting the group to your campaign
Before doing any of that, one would need to first discuss with the players and see if they have the same worries about the new group composition vs the “style” of the game so far.
Maybe they feel totally fine and feel like they can still prevail with the same level of difficulty you have given them so far. If that is the case, then keeping status quo seems like the way to go.
Or maybe they are worried that they cannot logically continue in this game with the group composition they now have vs the “style” of the game so far.
In this case, two choices: adjust the campaign or adjust the group.
First ask them if any of them want to roll a new character if that is something you are okay with. Of course this is not a very efficient solution if they end up replacing another important role, like the Wizard who is normally a heavy damage dealer or very efficient enemy disabler rerolling a fighter might not be a big help to the group’s balance vs the challenge rating of your encounters. If your game is not heavy on social interactions, then maybe the Bard would be the best candidate for a character change.
If they are attached to their characters (which is totally legit), then maybe you can better adjust the game. There are multiple ways you can achieve that.
Adjusting the challenge rating of your encounters is one. You might be able to lower that easily enough and still be able to give good challenges to your group.
Adjusting the focus of your game is another. What I mean by that is maybe you can change the focus of the game from “combat centric” game to a “political intrigue” or “horror investigation” game.
Giving less fights between rests is yet another way to do that. It ensures that your heavy hitters (higher level spell slots on your casters, mostly) can be used more frequently.
You could also make your fights more tactical. Makes your maps more tactical, so your players can exploit tactical advantages to their advantage. By allowing your players to exploit terrain to their advantage, by allowing them to gather more information on their enemies before they have to engage them, you tip the scale on their side, if they are smart enough to exploit those.
About status quo
Sometimes I would keep things as is, keep the status quo. It all depends on the nature of your game and imo is riskier to do in combat heavy (aka ‘difficult’) games. There’s always the risk of a TPK in those game, more so with a suboptimal group.
In a more combat heavy/difficult game, if I wanted to keep the status quo, I would sometimes create a NPC that would fill the missing important role for the group. Sometimes I would control it, sometimes I would give control of it to a player or the whole group. But beware, these kind of NPCs can quickly turn into cannon fodder, with certain groups.
What can be done, concretely, with this group composition to maintain the balance now that they lost their frontline fighters ?
You clearly seem more interested in continuing the game with the current group composition without changing the group composition and also clearly worried they might not be able to pull it off.
You can allow your players to “retcon” choices they had made previously, if some spell/class features/items they got would be better if replaced by others, now that the group has changed.
The Druid can be an efficient tank, with summons and wild shapes. I once was the main tank of my group (actually on my last PF game ever). It was not always easy, but it got a whole lot better once I got a magical armor that would still remain active in wild shape (I think it was called Darkwood armor) ... that did help a lot in giving me survivability. You should consider giving a magical item to boost the druid’s AC a bit if he becomes the main tank.
The Wizard should keep doing what he does and nuke the enemy as best as he can. A dead enemy is one that does not hit the tank or anyone. He might also want to get more crowd control besides Web. If you allow for more rest and refrain from racing them against the clock too much, then the Wizard can have fun blasting away every fight.
The Bard. I feel like he is going to have to switch his focus from constant, but probably low, damage from ranged attacks to more crowd control/disables and rely on his usual combo as a last resort. But I would also let him figure it out by himself.
The Oracle, is harder to size up. It all depends on the secrets/revelations and spell chosen. Some buffs for the druid, some heals, some damage will all be useful. Going from paladin to oracle, I’m guessing he has a plan.
I’ve mentionned magical items for the druid, but everyone in the group would benefit from those. I’m mentionning this possibility not because I think it’s the best idea, but you can pinpoint their main weaknesses and help patch them up with items, giving them more survivability.
I’ve tried to condense my answer, getting rid of some useless fat that you mentionned.
Good luck with your game !