I'm currently maxed out on legendary marks. What should I buy?

Until I ran out, I bought Year 2 versions of all the Exotics I had Year 1 versions of. (The kiosks are in the room with the Vanguard leaders.)

Occasionally, I'll buy a special weapon legendary engram from the Cryptarch, on the off chance I 1) get a thousand-yard-stare, and 2) get a roll that's better than what I already have.

If I'm at all low on planetary materials, I'll visit the Vanguard Quartermaster (in the hangar) and buy some.

If you haven't already, you should go buy the FWC Fusion Rifle; The Vacancy. It's a monster of a weapon. Combination of very high stability and great range, plus high impact. I'm consistently surprising myself by the range of the 1-shot kills I'm able to get in PvP.