I know sage mode this, senjutsu that, he must have it etc... But Monkey sage mode sounds pretty awesome. He mastered every jutsu in Konoha didn't he? It would give hiruzen something and god knows he needs something. I dunno if he could summon enma or if even summoning enma would be a good idea? He can't use fire style, shadow shuriken was useless, and without Enma he can't use that rod thing or whatever it's called? It's only logical for him to use a technique we haven't seen before. For the god of Shinobi Hashirama to have it. For the sage of the 6 paths to be a sage he should be able to do it and Naruto the future god of Shinobi is capable. Why not Hiruzen?