I'm pretty sure if Monero has a way to obscure transactions without anyone know who is sending transactions or how much they are, they can obscure the fees too.
Fees are a mechanism for letting miners prioritize transactions and weed out spam; thus the amounts need to be visible to miners.
With my design for a "blockchain", there is no such thing as spam. The more the merrier. The entire data doesn't need to be downloaded in order to prove a transaction. Miners aren't subjected to the entire volume of transactions.
Thus afaics the only reason to make tx fees higher than costs+competitive level of profit (or to make them public) is for some groupwise (collectivized) reason, such as needing to squelch the spamming of anonymity sets.