Well, as it turned out, I didn't need much of a change at all to make the encounter dramatic. I had planned a whole new "boss fight"--a Red Wizard and a few yuguloths staking out Chuth's lair--but I ended up scrapping it because the PCs were pretty tapped out by the time they got there, thanks to a couple of additions to previous rooms. (Also because it was getting late and I knew we wouldn't have time for another big combat that night, and we're going on hiatus until spring, so I wanted to get to a good stopping point.)
The additions I made were to add half-green dragon gladiators to areas 4 and 5 (one in each area), and mages to areas 5 and 6 (also one per room). I also changed the dragonclaws to dragonwings and gave each one (IIRC) 37 hp. They never got to do much damage, but at least they stayed up for more than one fireball and soaked up some party resources.
The PCs wound up in the Y-shaped passage fighting off everyone in areas 5 and 6 from both sides at once, including the two mages. That made for an exciting capstone to the evening. One of the mages got into a veritable magic duel with our wizard, using counterspell and shield against each other. I decided the mages were evokers (because they probably would be), giving them the ability to sculpt their allies out of spells. And the dragonwings kept our melee fighters distracted enough that the the gladiator stayed up for a while, and the enclosed space meant that he got to use his dragon breath effectively because the PCs couldn't spread out too much.
I made up the stat block for the half-green dragon gladiator by mashing together the Gladiator and Half-Red Dragon Veteran stat blocks, and then substituting the green dragon's breath weapon. Basically, anywhere the Gladiator and HRDV stats differed, I took the higher option.
Half-Green Dragon Gladiator
Medium humanoid (human), any alignment
Armor Class 18 (chain mail + shield)
Hit Points 112 (15d8 + 45)
Speed 30 ft.
STR DEX CON INT WIS CHA
18 (+4) 15 (+2) 16 (+3) 10 (+0) 12 (+1) 15 (+2)
Saving Throws Str +7, Dex +5, Con +6
Skills Athletics +10, Intimidation +5
Damage Resistances Poison
Senses blindsight 10 ft., darkvision 60 ft., passive Perception 12
Languages any one language (usually Common)
Challenge 5 (1,800 XP)
Brave. The gladiator has advantage on saving throws against being frightened.
Brute. A melee weapon deals one extra die of its damage when the gladiator hits with it (included in the attack).
Actions
Multiattack. The gladiator makes three melee attacks.
Spear. Melee or Ranged Weapon Attack: +7 to hit, reach 5 ft. and range 20/60 ft., one target. Hit: 11 (2d6 + 4) piercing damage, or 13 (2d8 + 4) piercing damage if used with two hands to make a melee attack.
Shield Bash. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 9 (2d4 + 4) bludgeoning damage. If the target is a Medium or smaller creature, it must succeed on a DC 15 Strength saving throw or be knocked prone.
Poison Breath (Recharge 5–6). The gladiator exhales poisonous gas in a 15-foot cone. Each creature in that area must make a DC 11 Constitution saving throw, taking 21 (6d6) poison damage on a failed save, or half as much damage on a successful one.
Reactions
Parry. The gladiator adds 3 to its AC against one melee attack that would hit it. To do so, the gladiator must see the attacker and be wielding a melee weapon.
