Huh. That sound true, "what's hot in finance now won't be by the time you finish your PhD".
Well, how about you try to think about what the world will likely want in 5Y. See what the macro-economists are saying. Like for example, if in 2008 someone was thinking "Ok, there is a global financial crisis. The underlying cause is so fundamental that it's not going to go away. Banks will be hesistant to give loans. There will be super-paranoia of default. Also things might get so bad that some countries are going to be so in debt that people are going to find it ridiculous to still have a currency linked to them. So maybe I will work on credit default swaps and other credit derivatives, or maybe bond pricing or bond hedging or any bond-related derivatives, or quantitative risk management."
That is the idea: You read the general consensus of the economists. You then think about what kind of economic climate that entails. You then think about what specialization would be useful in that economic climate.