We do have random invasions, and semi-random ones.
I totally, COMPLETELY agree about hamidon. I want to go back to the old Hami raids. I don't like the new ones. They're sad and boring.
As far as challenges, though, I solo - I don't need bunches of ways to make my character die - and there ARE plenty of those kinds of "challenges" in the MA. I grade those poorly, 1 or 2 stars if I have to keep running back from the hospital. Heck, the posi task force I would grade at 1 for that reason... 
Protector has an event every once in a while, called "Robot Rampage". You can spawn the Kronoses using the Oro missions, and Babbages, and if you time it right, head over to Kings Row when there are a couple Paladins walking around. It's lots of fun! And, we can do it ourselves!
But I agree: I like having lots of things to bring folks together. I remember doing Lusca raids, and it was one of the few things that I was really good at with a new team or even a bunch of strangers.
Please read my FEAR/Portal/HalfLife Fan Fiction!
Repurposed
Excuse Grammar and other mistakes please, I'm French^^)
I'd like to ask Dev to give us more great stuffs like Hamidon, where so much players had fun to participate.
Let us do Hamidon back with about 300 players... Give us other stuff like that please...
And for future Issue, what about to put great awards on those stuffs...
For example 1 more enh slot achivieng a really hard Trial.
We need more challenge.. We are Super heroes/Super Villains
Architect is great but will not renew interest on many players cause there's no more challenge on Architect than on some TF.
And I repeat... Heroes/Villains need challenge... or Warrior's rest will be change to R.I.P for them
I see many players who don't have interest anymore with their 50's in spite of Architect...
Why to wait a special event to make many Giant monsters spawn on main areas ? Random this stuff on a month maybe !