I don't think that is something to worry about. Reeves found a way to give Catwoman a whip, so I can see Oswald gaining some form of umbrella weapon either here or in the future. Reeves clearly has more appreciation for the iconography and tropes of the comics than say Chris Nolan. However, in accordance with Reeves' gritty noir aesthetic for this film, I wouldn't expect really heightened, fantastical umbrellas like what Burton gave DeVito. I would expect more of a single shot concealed umbrella gun (like the classic cane guns from the 19th century) or a play on a concealed sword in a cane from the same period.
Oswald is definitely going to be "fun"... Reeves seems to let Farrell chew scenery and channel his inner Joe Pesci/Tony Soprano, and I expect the character will be have lots of the black humour for which those characters/actors are known.