Is it actually possible for them to keep adding content without raising the level cap

[BlackShroudMercs]

If they keep going with item levels, and stack content based on item level, add quests that are accessible based on item level etc. it could get pretty interesting.

How do you guys think they'll handle it?

This is how DCUO does it, almost all 2/4/8 man content has an item level requirement.

This keeps a MMO from raising its lvl cap, it is wise considering IMO, raising a lvl cap in an MMO is one of the last things you want to do.

I don't entirely disagree with the methods used. Players without the required gear are kept from dungeons they shouldn't be in, that's a good thing.
On DC, your overall item level is refereed to as CR, so normally shouts for a role would have a required CR number attached, you'll see that here soon.

As for the whole item lvl vs lvl cap and how they affect content well, that's debatable for sure.