How do you decide what to include in a launch or tie products together if they're not necessarily around a common business problem or for a common persona?

There's always a way :) 

But if that's the case, you have a great probablem. Why not separate and do two launch moments? Keep the drumbeat and make it easy by sticking to one key message for each launch.

If combining products or news together feels forced, then don't do it. Your audience will notice, and will get confused.

This is something I deal with a lot. Often, for a large launch, you’ve got multiple product components that you’re bringing together for one big moment. I find that launch teams often have that "more is more" mentality, where our product teams push to get something else completed with the energy of the launch. While I love the energy, all these components can be impossible to tie together and manage as you’re getting close to the launch. As a rule of thumb, I focus on the customers we’re targeting and the story we want to tell as the two lenses I use to decide what to group together and what to create a standalone effort. My biggest suggestion for managing this is to get in front of it as best you can. Here’s how I do it:

• Yearly planning: As we go into our planning for the fiscal year, I sit down with our product leaders and map out the major launch moments for the year. We look over the proposed roadmap and talk through what we think the big launches will be for the year and the possible components we can add to the launch. From there, I drive an exercise focused on determining what we think the headline of that launch could be and the main perceptions we want to drive. Once this is done, we look through those components and determine if they strengthen the story. If they do, they’re in. If they don’t, they get moved to a standalone, smaller launch. This is a great time to get your product members on board with what the launch could be, and you have alignment on what to do as new product opportunities pop up and the launch gets closer. It's also a great way to make sure your key teams, like creative, content, and web know when big launch moments are coming so you're not catching them off guard with your requests. 
• Quarterly planning: With our big launches mapped out, we go into each quarter looking at what we think will be coming up in the next 3 months and how to plan for those launches. We decided to group things together if they strengthen one another’s story. Sometimes we’ll also consider combining if they’re happening at the same time and we need to combine to save team resources or not over-contact our customers…but only if we can find a way to tie them together that's intuitive and will make sense for the customer. If that can't happen, we'll hold the launch for a week or two to give each product development its time in the sun. 
• In launch prep: Often, during launch prep you realize you have WAY too many things going on. At that point, you sit down and map out what should be held for a follow-up launch moment. The story you want to tell and perceptions you want to drive are key in that decision-making process. After our last big product launch at Airtable in early November, we did 3 “follow up” launches focused on features that launched with our large launch but didn’t get much airtime because they didn’t fit with the story.

Whoever asked this question: I suspect at your company there is opportunity for a tighter partnership between product management + product marketing that goes beyond only coming together for product launches. 

You wouldn't have asked this question if PM+PMM planned product strategy together and looked at the roadmap earlier on. You're asking this question because things currently work as a hand-off process, and PMM tries to "work with whatever's been given." This is not an ideal, long term setup, so I'd recommend to fix that asap.

Now, you need to work with the hand you're dealt. Tier the products/features to see if that helps to shape some sort of common value for a persona or industry. If that doesn't help, then force yourself to pick 1 product/feature to build a launch theme around. Everything else is gravy, but what's that killer product/feature you can confidently ground the launch on? Ruthlessly focus on that to hook the audience. The others hopefully help your audience stick around.