For a 51% attack as of right now:
I have no concept of the power of supercomputers so its really difficult for me to get hard numbers on this. Is it possible that they could create a botnet and attack that way? I know that Monero is probably more susceptible than bitcoin, but its hard to gauge since they both have different mining algorithms. Is it not even possible? Are the incentives just not set up in a way that would make sense for an attack like this?
For a community attack right now:
How difficult would it be to infiltrate the core team and get malicious code pushed into the next release?
For a Sybil attack right now:
we saw this a couple months ago and it seems to have gotten mitigated, how difficult was it to pull this off in the first place? Can it happen again?
Is there another attack vector i'm missing?
If anyone has other attacks in mind comment on them, i think its a very good idea to think about these things.