1.Okay, firstly, you want to redo the area, in my opinion, inless there is a complete reason for having a random volcano area there.. Basically that room you've made looks like it was made with a volcano inside it, which is unrealistic, make the room look like it used to be something else, and the volcano has arisen from the ground, for example, make it look like it used to be a prison room, and the volcano has burst up through the ground in the center of the room.
2. Secondly, you don't want to use the 'Volcano' doodad, it sucks (No offense) but it just doesn't look that good at all, Use the 'Raise ground' tool and the Lava floor tile to make your own volcano, and variate it along the sides, Volcanos are never the same each side, they variate.
3. Use Shift when placing things like rocks, this reduces they're pathing size to 1 cell, meaning you can spam them more, without effecting In-game pathing. Also use Alt+Page up and page down, to change the height of things like rocks and rock archways to hide the ugly bits at the bottom.
4. The lava on the floor would generally be bulgy, and would be in lumps and clumps and at different height levels. So when using things like Rocks near the edge of the lava, you want to change the Max Roll angel and the Max Pitch angel both to 90.00. This will make them adjust to the different angel of the ground, making the place look alot more realistic.
5. Rocks Would be placed more towards the edges of Lava as this is where the lava would harden and become Volcanic Rock, I.E Your Rock doodad.
6. Lava wouldn't generally be in a pool as such, it would flow in streams to the more Downward place in the area, so there would be several big pools of lava, inless your volcano was Very Very active in which case there would be a giant pool, but from the size of that volcano i don't think there would be.
7. Also there wouldn't be as much fire, fire comes usually from lava striking woodland or objects that are flammable, ManMade walls and ground tiles aren't flammable, but there would be some fire from escaping gas which would be igniting, but there wouldn't be loads of it. Also i would get rid of the Fire trap doodad things cause they look really ugly in my opinion.
Here is something i did quickly, I haven't used varied floor tiles, as i will leave that for you to decide. It's just as an example of what i would've done, its a room that was used as a caged prison, but the volcano burst up through the center of the room, with lava pouring out of the volcano, No custom doodads are used, no custom tiles or skins or anything is used, its just pure Warcraft 3, it's just an example, which is fully Playable.
Thats about it. Sorry if it was a bit blunt, I'm just giving my opinion.