Looking at what's out there right now, I see Fundition as having a lot of room to grow. There are so many people who love donating to charities, but just don't have money to give. The ability to simply upvote the charities you care about and essentially have someone else's money go to them will be a draw to a lot of people. Word just needs to get out and everything about joining and using the platform made easier to understand by non-crypto folks.
Steempress also offers a lot of potential for growth, because you just install a plugin and your normal blog starts earning you money on this other platform. Again, training is the essential issue. I find the Steempeak interface to be easier for newbies than steemit, so my plans for a training program I'm putting together for teens on nearby reservations will involve that combination.
And finally, I do see a lot of potential for DLive. I'm trying to get myself using it. Just had a lot of technical issues when I was only on mobile and now that I'm back on laptop simply need the time to give it a try again. I used to do a lot of livestreaming (even way back when Periscope was just starting), and I know that's a way of communicating/receiving info a lot of people really eat up. Being able to directly monetize that will have a lot of appeal.
As for yet to be created uses, well I think you pointed toward the big one in your OP, the ability to pay for things with STEEM even when it has nothing to do with content creation. To me that's the game changer.