There are a lot of variables in your situation that I don't know the answer to and so I'll try to keep my suggestions as high-level as I can. Either way, please use them as only a starting point.
5 Levels of Planning
First I'd look at the 5 levels of planning. There's a lot of info to look up on this, but the short version is that they are:
Daily Planning (Scrum)
Depending on the length of your workshop, you're looking at Vision and maybe some roadmap.
Product vision is just part of this. If you haven't used other product vision approaches before, I'd look at the elevator pitch format and the product box exercise (design the imaginary box your product will come in). These are great ways to get ideas flowing. The second option could easily be a workshop in itself since it'll get into things like key features and constraints.
Vision planning may also include things like stakeholder maps, constraints, and personas.
There are a lot of ways to approach this. Two good options are:
Lean Project Canvas to prioritize major features:
Full disclosure, I know the author, so I'm not unbiased here, but it is a technique I like and use myself.
Short version of this is you brainstorm major features and use these canvases to give them a good apples-to-apples comparison for prioritization.
There's a lot on the internet about story mapping and it would be ungainly for me to try to describe it here. Going through a full-blown story map here is probably taking it too far. The narrative aspect of a story map is what I'd focus on. How do they see a customer going through the major features? This can also act as a foundation when you bring the other teams in to start digging in.
I hope this helps. Start with that 5 levels of planning idea and you'll find tons of other approaches out there too. For the most part, they're all good. Just find the one that fits for you.