So, I've been playing around with the scenario options in the map editor. You can do a lot with triggers (and I intend to). However, a few additions to the trigger system would really open things up!
(1) Triggers that turn off other triggers. Right now, triggers can turn on other triggers, but I can't see a way to make them turn them off. Making it so one trigger could turn off another would allow for some really interesting interactions, and could be used in myriad ways. This is the big one.
(2) Random chance triggers. Give a trigger a chance of happening. Right now every trigger is 100% likely to happen, which creates a very deterministic world. Players that memorize the scenario triggers will have a big advantage, and will know exactly how the scenario elements will play out every time. Triggers that can turn off triggers could be used to mix things up a bit, but that would be a bunch of extra work. It would be nice if we could just have a percent chance.
(2.5) This trigger OR that trigger. This naturally evolves from (2) above. Give a trigger a % chance of happening, but if it doesn't happen, allow it to activate another trigger. Could be use in MANY ways.
(3) Move stack trigger. Take the units from one location and drop them in another. This trigger would make teleporters, tunnels to underground realms, and other weirdness possible.
(4)Triggers that ask permission. Add 'accept' and 'decline' buttons to a trigger that creates a pop-up text window. The 'accept' button would activate the trigger's other effects.
(5) Triggers that change terrain. Might be tricky to code, but these could be used to create seasons, flooding, volcanic eruptions, and wall building (just to name a few uses).
(6) 'search a ruin' trigger. Why not? I can think of some fun ways to use this one.
(7) Triggers that steal your gold. You get the idea.