I would have to say for me the biggest concerns for bitcoin would be..

1. The Darkweb gets shutdown or takes another hit. IMO the Darkweb/illegal activity plays a huge role in the success and failure of bitcoin.

2. Banks getting involved. If Banks are losing out on money or think bitcoin is taking money out of their pocket they could create a mess for bitcoin.

3. The richest of the rich will own most bitcoins. The rich usually make money from the middle/lower class, if they own most they will be no profits for them and bitcoin become obsolete.

Just a few of my concerns and obviously just my opinion. If you have any concerns or comments I'd like to hear your input.