"Barley is the grain normally used for beer "

It still is.   There are other adjuncts to beer making, such as wheat and rice, but those are primarily flavoring agents - not for alcohol.

Alcohol is produced by the fermentable sugars in the beer. You could fill it up with table sugar, and while you might get higher alcohol you would also get a terrible tasting drink.

By the way, wheat beer is an exceptional style. There are some lovely German wheat beers that I highly recommend.

This discussion has got me itching to do another batch of home brew!