Finally there is a trend of using recurrent neural network as a top component of the Q-network. Perhaps we will see even more sophisticated RNNs like DNC and Recurrent Entity Networks applied here.
Also we'll see meta-reinforcement learning applied to a curriculum of environments.
That paper was 10 months ago. There have been many RL papers in the meantime, but sparsity is only a problem with respect to reward, not state or action, from what I can see.
https://arxiv.org/abs/1602.01783
Is there a way to deal with "sparse" training data (state, action, reward) triples -- sparse in "state"?