Some effects could be interesting, like the synergy between minions and other traps. Minions always felt a bit disconnected from other traps for me, as their health and damage don't seem to scale well against higher waves (though I've never tested it).

Originally posted by Turbo-Laser Ninjadragon:

Demons+Ice=Slowed and weakened Demons

A syngery that weakens a trap(s) seems pretty counterproductive though.

Overall... I don't know, most of these ideas seem to be overly flashy. Effects like a "pulsar" would likely be more complex to calculate and put a bunch more of effects on the screen, which impacts performance at higher waves.
Flame darts could just be another dart trap upgrade without the need of placing a flamer nearby.

A "superconducting cathode ray emitter" would... what, slow enemies in an area and do lighting damage? Seems like a straight upgrade to the freeze trap, whose only innate damage source is the damage upgrade based on its mastery points.

Traps already have synergy by affecting enemies. Making them depend on each other like this would make them compete even more on the tiles of a map, in my opinion.

Just an example of a way that it could be balanced, not an outright suggestion. Really there are lots of ways Synergies could be prevented from instantly becoming over powered.

Like:

Fire+Demons= Demons heal faster...but move slower
Electricity+ demons= Demons move faster but heal slower
Ice+Demons ....Honestly makes almost no sense to me ice and Demons are like oil and water but of course it's not up to me it's up to the designers and Probably they wont implement any of this because the game is more or less complete...it would be supper cool though.

here's what I think: In an interview with Markus Persson (notch) I once watched he was asked how he ever came up with his design ideas and his answer was just that every time he thought about a feature he just asked himself "does it make the game more fun" and that's all he did. So I think , any idea is a great idea but at each point "is it fun" is the most important thing. I think trap synergies are a cool idea, there are lots of places where they are more fun but there are some places where they wouldn't be...so...don't include those.

:)