Fullmetal Alchemist, Toaru Majutsu no Index Recommendations
Both series deal with mature teenagers going through a great struggle to achieve some bigger goal, going to particular adventures, fighting for their lives. All the hardships they go through, together with friends and comrades, is in both series almost similar. Sometimes the characters are a bit more childish in FMA², but both series deal with alot of the same elements.
Both anime tell about supernatural powers. Fullmetal Alchemist is about alchemy and Toaru Majutusu no Index is about superhuman powers and magic. There is also a little bit alchemy in Toaru Majutsu no Index which makes these anime pretty similar.