The Game Plan
Flare is only going to be relevant if I release several games: improve the engine each time until it's proven and practical.
I'm going to save the new game scale for game #2. For now I'm going to take what we already have and think about the kind of small-scope game that can be made. Maybe grasslands, caves, dungeons, and that's enough for one small episode.
Here's a tiny story I could tell using most of the current art.
Part 1: Mines
Trade caravan drops off the player and supplies to a mining outpost. Player's just looking for work. Some of the other workers are sick so you're a welcome addition. Take a pick, head down into the mines. The hero follows a suspicious tunnel and ends up in a creature-infested cave system. Has to fight his way back to town. One of the miners died, became a zombie, and had to be put down. This is trouble.
Part 2: Journey
The village will be wiped out without help. There is a fort not far from here that houses an old paladin order. Maybe they can send swordsmen or magical help. Travel overland to the fort, only to find the area crawling with creatures. Help fight the undead. Help find ingredients for a possible cure.
Part 3: Passage
The villagers must come to the fort for safety, but overland is too dangerous. The paladins know of an abandoned tunnel that connects the fort to a spot near the village. Clear it and get to the villagers, and guide them to the relative safety of the fort.
Part 4: Siege
Back at the fort, the villagers get help. The potions seem to make them feel better. Meanwhile there is a heavy siege under way against the fort, an army of undead. Fight against the hordes at the gates. Find the twisted villainous leader and destroy him.
Part 5: Guardian
When you slay the villain, his beastly corpse becomes the body of a frail old druid. The paladins recognize him, he was a great elemental guardian. Something or someone corrupted him, allowing evil to overwhelm the scales of balance. The player becomes the new Guardian. Some of the previously-sick villagers and paladins, slain in combat, are becoming undead. The cure has failed, only treating the symptoms. When you all finally die, you will become undead too. As the new guardian, you eventually seal the Fort with yourself and the infected survivors inside.
Most of this short story can be told with the existing assets. Let me list out how it would work.
Part 1 would use the grasslands and cave tileset to represent the mining village area and the mines. The caves could contain goblins, antlions, and a couple zombies.
Part 2 would feature mostly grassland areas. Closer to the village there can be more goblins/antlions, and closer to the fort there would be mainly undead. There would need to be a new set piece for the Averguard Keep entrance; it doesn't need to be a whole tile set (it can be e.g. a mostly pre-rendered set). The interior of the keep would be the dungeon tile set.
Part 3's passage would be half dungeon, half caves. The dungeon half of the passage could be minotaurs, and the cave half could be tougher antlions.
Part 4 would again happen at the gates set piece, probably an instance map that now contains tougher undead and siege tiles. The boss encounter could be just inside the fort, perhaps. The boss itself would require a new creature model.
Part 5 is mostly the denouement. Will require a handful of new graphics but nothing like a whole tile set or creature.