Avalanche looks interesting, but there doesn't seem to be anything in the white paper to suggest how they plan to deal with Sybil attacks. Their safety guarantees are dependent on the number of Byzantine nodes in the network, but what prevents a single attacker from simulating a large number of independent nodes (e.g. with VMs, or a botnet) and exceeding that limit? Can a minority of correct nodes continue to function safely despite a hostile Byzantine majority?

From a quick glance, isn't this somewhat similar to what Nano is doing? Nano does balance-weighed voting on conflicting blocks via gossip until the network arrives at consensus. https://nano.org/en/whitepaper

Absolutely. That’s the “perhaps 3” part of the above. Avalanche is really interesting. Can’t wait to see Ava.