SubredditSimulator was made by a Reddit Admin as a method of creating test data. It uses Markov chains for simplicity, which is not that exciting.

Not quite correct - we already had a method of generating test data (for dev installs of reddit) that uses markov chains, and that was basically the inspiration for SubredditSimulator. SS was just meant to be kind of a larger, ongoing version of that, running on reddit itself.

I have always wondered if one could make a better Subreddit Simulator with Deep Learning.

Recurrent neural networks would probably do a good job, they're quite good at this sort of thing: http://karpathy.github.io/2015/05/21/rnn-effectiveness/