So outside of a core set, we're primarly interested in playing the Delve mode (We don't have the opportunity to engage folks in the campaign modes). Is there anything we can specifically do to inject a ton of variety into Descent with the Delve part of the app?

Boxed expansions work really well here. Items, classes, Heroes, monsters, boss stages, normal stages, tiles, conditions all make the boxes expansions great. I would favour the expansions with classes, delve gives you great opportunity to try it all of them.

Boxed expansions work really well here. Items, classes, Heroes, monsters, boss stages, normal stages, tiles, conditions all make the boxes expansions great. I would favour the expansions with classes, delve gives you great opportunity to try it all of them.

I'd have to second this for the (current) campaigns you're much better off getting H&M packs, but for the Delve, boxed expansions definitely give you the most bang for your buck.