The difficulty with pickup games is knowing what role you are supposed to play given that you have not had any prior contact with the other players, as well as having the potential for hero duplication which may throw a monkey wrench into your own deck strategy if you have to switch out. So my question is how to build a deck for cooperative play that I can build that does not rely to heavily on the heroes so if I have to switch one of them out, there are alternatives I can use or that it would have minimal impact (tall order I know, given that decks are built around heroes). I am not necessarily looking for decklists though they might help, rather I am looking for advice on what themes, spheres, I should include in this deck. I should add, that I am primarily a solo player, so it is difficult for me to think outside of my own sandbox.

I think mono-sphere is the way to go. Most of the vetern players like Ian bring a mono deck (I think he goes mono-leadership). That way, if you need to swap out a hero its not as big a deal and on top of that if most people bring two mono decks to play, they can usually get into a game very easy if others are doing the same.

Yeah several suggestions from what I've heard people say and what just makes sense:

Ranged and Sentinel help with combat across the table

Spirit is pretty crucial to cancel Treacheries as they become more punishing in multiplayer with so many cards being revealed (Eleanor is still great)

Having heroes with abilities that are better in multiplayer: Fatty can raise your threat to save several other players from raising threat.. Brand can be used more often to help readying

Having good support cards is also nice so bring card draw, healing, threat reduction that you can play on others

Remember global buffing cards or ones that can help many people: Dain, Mutual Accord, Campfire Tales, Light the Beacons, Grim Resolve

Try not to use heroes that need specific cards in case you need to swap a hero out in your deck

In general just think outside the box of single play, pull out cards you haven't used, ones that will seem better in multiplayer and whatever can be helpful to several players.

I think the best thing is to bring two decks: one "questing" deck and one "combat" deck - that way the two major bases are covered, and whatever your partner's deck is weaker at, you can fill that gap.