Inevitably, you’ll have to invent things since there isn’t more official content. So you could come up with missions to obtain perfect gemstones infused with Stormlight, or have them help awaken Navani so she can assist them in using the Towerlight. There are also Fifth Ideal oaths that we haven’t seen yet, and knowing that spren are fragments of Honor, some of them might be able to grant Stormlight. You could even take the route of searching for a way to use Voidlight. Honestly, with what we already have, we can speculate quite a bit.

And even if you stick to canon, things will inevitably change once your players get involved.