Two things:

1. Figuring out how to properly check for "no change" transactions [0] when addresses eventually loop back.
2. Coming up with a scalable way of storing asset transfers for Barreleye [1] using Clickhouse.

[0] https://en.bitcoin.it/wiki/Privacy#Exact_payment_amounts_.28no_change.29
[1] https://github.com/barreleye/barreleye