I've been giving this a good thinking over. Ever since my titan got to 34, I've had 200 vanguard and 200 crucible marks for a few weeks now.
With the recent expansion coming out maybe this will change, but here's what I think we could do with those:
Due to not upgrading more guns and armor with less then sufficient stats, I was thinking marks could be used like weapon parts to modify existing guns and armor.
What I mean by this is simple: You land at the Tower. You show up to your favorite vendor (FWC, DO, NM, Vanguard or Crucible masters) and open up the menu. There is three new items. For now, we'll just call these items compact.
Compact A is a special kind of scope you get and when combined with a gun? Gives you a random list of options to choose from. You can now select the three kinds of scopes you want. Didn't get what you wanted? You can always go get another. Cost on this is 3-5 marks for one.
Compact B is a generalized stat modifier. You can select one and it will help increase that stat percentage up to a certain point OR you can weaken a stat percentage with help from a third compact detailed below to do more things with this like push another stat to new heights or "beyond max".
Compact C is a switch modifier. What this means is you can combine this with another compact and it will grant you specialized access to different things. Take A + C. Instead of a scope change? You now get to do this for either: Two stats of your choice OR a drop down tree of random perks for the gun. Take B + C? You can now modify all gun statistics OR make a gun more powerful in the sense a 331 would go to 340 up to the current max.
I leave prices for this stuff based on you guys, but the idea here is about creating more options for our guns. The same goes to our armors too.