This is my idea. During times like X-Mas, Halloween, etc. an update gives a special item to every 5 levels.
Example-Halloween Event
Level 5: Pumpkin Rockets
Level 10: Candy Machine Gun(suprisingly powerful, also distracts the enemy from eating candy for 1 round.)
Level 15: Spector Hook(Pulls enemy over, then knocks em' back with a powerful Ghost.)
Level 20: The Dutchman's Fury(A pirate ghost appears from a ship in a bottle, and haunts the enemy, making him parylized with fear for 1 round. Mechs are machines that are controled by people inside you know!)
Level 25: Mechenstein(Special item like Dutchman's Fury or Spector Hook. A mech version of Frankenstein attacks the target.)
Level 30: Monster Mash(Special item. A swarm of monsters run through and attack the enemy.)

What do you think? These items would be limited time only, and would be removed at the end of the event, but you would still have the item.

They all sound extremely powerful which wouldn't be a good thing possibly since players that come after those events would be weaker because of it. Don't get me wrong they all sound pretty cool but if they were too powerful it would be a problem then there is also the issue of our devs having to try and code, design, and balance them in a very short time before the holiday. Maybe this could be done something a bit farther out say christmas so they have a bit more time but for now players could maybe receive an aesthetic gift for the holidays.

It was just an example. That exactly didn't have to happen. And the point is to give the players something a little more powerful to make them feel special. Also, the weapons wouldn't be to more powerful than the regular weapons of they're level.

I think things like special mech "costumes" or simply visual changes on items already in place would be good. For example for halloween give players a skin to make the hook look like a jack o lantern flying out biting the other mech and then pulling it back.