It's a toss up between making things burn or explode...it has to be explosions all the way.
So many ways to make things explode or burn, often at the same time too. ;)
I picked elixir gun because it's balanced with some damage and some healing, but frankly, I'm likely to be flipping between all of those choices constantly without having clear favorites.
Mines. Sounds fun having to micro the same skill 5 times, keep track of what is where. This kit will be difficult to play so most likely have many buffs in the beginning because scrubs whine. Since buffs are never ever reverted, this can become the staple kit for a skilled player, ie me.
For example enemy team comes up to a keep. Line up mines to bomb the trampling zerg one after another so they are heavily damaged even before they arrive. While your team gets pushed back, mine several vantage points behind own lines and switch to another kit to help out. When your line folds and enemy gets into position, blow enemies as they are getting into a safe position.
In 1v1 gank, run away and put mines down. He can follow you but this would mean he's already damaged at which point switching to flamethrower would be pewpew. Or keep throwing mines while going to point A to have a fall back route for when unexpected things happen. This kit looks really versatile while just providing a single skill in essence.
Does the mortar count as a kit? I guess not really.
It's explosives (grenades) or bust.
I'm actually kind of surprised by this. Most of the time when I play Engineer type classes, I lean towards gadgets and bots of some sort.