Just a few upgrade and enemy ideas I had, to fill things out a bit more.
Abilities:
1: A deployable energy shield that blocks all arrows/hammer/sword attacks in the direction you're facing, perhaps sharing its radius with the upgrade level of arrow blocking you have. Downsides: You can't attack while it's up, and it drains energy at about the rate fire breath would. 1-2 tiers, maybe? Unless it was tied in with arrow deflection.
2: Rocket feet. Essentially either a high jump (about 2-3 robots high) or a double jump that allows for a bit more maneuverability (could also lend itself towards parkour challenge maps). Could consume energy- and perhaps even a final tier that lets you continue to jump indefinitely as long as you have energy past the second jump.
3: I dunno how this would work as an actual ability but a way to "kick" or otherwise knock robots upwards. Maybe an uppercut? Might be tough to work with since we use both hands for most weapons.
4: A spear, or perhaps even a trident weapon used for mid-range poking of enemies. Wouldn't be able to cut huge holes in enemies like the sword or bow can but you could tactically disable enemies or poke at Spidertron's weak points with it. Could also be deflected by swords.
Enemies:
One thing that I'm a fan of the idea of is enemies that are only disabled when you attack a certain part, so for an example of that I was thinking a level 2 (or 3, if level 2 kick bots are on the way already) kick bot that only dies if you cut off a leg as opposed to its torso / head. In-game excuse could be that it has its brain functionality in its legs for improved kicking power or something.
Spear bots, using the same weapon I mentioned above.
Acro-bots that use the rocket feet I detailed before. I imagine they'd either use a basic sword or just heckle you like kick bots do.
Mark 4(5?? Are the fire ones mark 4?) archer bots with energy shields that come up every so often. Could just be a shield that blocks arrows if that would be too big a bugger on balance otherwise.
A couple of misc suggestions: Make it so that if you completely disable Spidertron's cannon / sever it from its body / just plain destroy it it stops firing. Also, maybe some benefit to cutting off sword robots arms?? Like making them not be able to swing in the direction you cut the arm off, or at least not as fast.
Commentary based on things happening in-match (deflecting arrow kills, having robots kill each other or run into lava/flame jets/etc, stuff like that) or upgrades you take could be neat, too, if those aren't too hard to come up with.
One more minor suggestion: Water. Placeable like lava blocks, and slows you down while you're in it. MAYBE, if it's not too hard to code, it could damage already-damaged limbs too- Under the logic of the exposed electronics getting water all over them and getting messed up.