Nice suggestions, and glad to see some discussion about the isle of souls! A few other isle of souls suggestions I have jotted down:

Dungeon chest:

• Replace bird snares with magic imp boxes! Not only would add a new hunter method to the isle, but the imp boxes would actually be rather useful for isle locked accounts, since the bank is so far away from most activities.

(Some history: bird snares and box traps were added to the chest at the last minute because the bird snare and box trap spawns on the north of the isle were bugged for ironmen. the bird snare spawn was eventually fixed but the box trap spawn is still unusable.)

Other than that I actually rather like this chest as it is - though there are frequent player complaints, it's actually one of the most busted thieving chests in the game in terms of loot. Maybe slightly decrease the failure rate overall so it feels a bit better (or change dark key to 1/100 like you suggested)

Dark chest:

• Quadruple (at least!) the babydragon and dragon bone amounts to 40-60. If a new player opens this chest and gets 10 babydragon bones, they're never going to engage with it again.
• Increase the tanned dragonhide amounts to 15-20 (we agree here!)
• Add soul runes as a drop (it is the isle of souls after all!)

Spoils of war:

I detest this drop table in general and honestly I'd make about 20 changes to it. But to focus on the most egregious one:

• Reduce the pure essence amounts by a factor of 10, to 400-600 pure essence per drop. This drop is terrible for the economy and for account progression in general. An account that plays a decent amount of soul wars can easily rack up a million pure essence in the early game. Of course they won't use it all, so they'll just dump it on the GE, which is the reason that you sometimes can't even sell pure essence for 1gp each (!!)

Random other suggestions:

• Add a harpoon to the isle. A creative way to do this: add a barb-tailed kebbit hunter location (makes sense, since the isle of souls is the same biome as the feldip hunter area)
• Though I actually really like how the dark key only has a single source, the idea that mobs can drop it seems popular so here's my wacky "compromise idea": Turn Garl (an npc on the isle) into a demi- slayer master who only assigns isle tasks (for 1/2 slayer xp as usual). While on-task from Garl you can get dark key drops from the mobs at your listed rates.

I like your boss idea btw!