Even if you manage to see an invasion there are only so many mages and healers.
I suspect we'll see more mage and healers in the event content as the event moves on. I read somewhere on stratics that the devs were primarily focused on fixing problems in the halloween content right now because that content ends sooner.
I suspect the invasions will get more love after the halloween content ends.
I play on a low population shard, so I have been able to see plenty of invasion content. The downside is that once the invasion is complete and the crimson dragon spawns, we often have to kill it with 4 people (less sometimes)