The ability to "star," or "favorite" the conversations that matter to you most.
This is another feature Slack already has that iMessage should have.
Since everyone at Tech Insider uses Slack, there are tons of channels dedicated to everything you can think of: Individual verticals for each subsection, channels dedicated to giving "props" to fellow colleagues who do good work, channels for just editors, etc. Add those group conversations to the individual direct messages, or private conversations you'll have in Slack, and you might feel a bit overwhelmed.
But when I open Slack, I don't feel overwhelmed with the number of conversations and channels I'm in because Slack lets you save your "starred," or favorite/most used channels, which are presented at the very top of the app so they're always easy to find. So even if you're involved in hundreds of different conversations, you can always find the channels that matter most.
This would be a great feature in iMessage, especially since the only way your messages get sorted is by recency. I would love to "star" conversations with my girlfriend and family members, for example, so it's always easy to find them later at a glance.
The ability to pay people using Apple Pay.
It's become incredibly easy to pay friends and loved ones back for stuff like groceries, rent, taxi rides or restaurant bills. Venmo is one of my most-used apps: My girlfriend and I constantly Venmo each other for all of the above reasons.
There are other similar services on PayPal and even Snapchat. But it would be super easy to port all of these features into iMessage.
How great would it be to simply send, receive or request money over iMessage? Apple already has a fully encrypted system for payments, Apple Pay, which is protected by your unique fingerprint and/or a four- or six-digit code to charge your credit or debit card on file. Adding Apple Pay to iMessage would take little effort on Apple's part, and it would make peer-to-peer payments so much better. This way I wouldn't have to download and use a separate app just for this one function.