Yellow Lantern ring. If there's anyone on earth better at using, creating, and controlling fear I don't know who it is.
Miraclo is a good idea, if the addiction problems have been dealt with. Not gonna make him a heavy hitter but it'd be more than enough to deal with guys like Croc and Bane.
Maybe he could start hearing and talking to the city like Jack Hawksmoore?
Honestly he could just use some Ted Knight cosmic tech, or stuff from any number of villains the League has taken down. Kanjar Ro had a pretty powerful energy weapon as I recall, that's probably collecting dust in some JLA storage locker.
If you really wanna be crazy with it, you could always adapt Warworld from the Young Justice cartoon and have Bruce wear the crown. For those unaware, on YJ the Warworld is a fully automated battle station and every facet of it's functions are controlled by a crown. Bruce could be in charge of a weaponized planet, and finally be a planet-breaker like Clark and Diana!
Of course PKJ just finished an amazing run with Warworld that wouldn't really fit the YJ version and it's not worth losing just so Batman can be even more badass.
But I always thought that if Bruce was going to get powers, which I don't agree with and don't want to see happen, it'd make sense for him to lean more into the supernatural than the superhuman. Making him a vampire is super stupid obvious, but you could also give him some more "stealth" based abilities and look to the old pulp heroes for inspiration; the Phantom, the Shadow, etc., those guys dealt with some occult powers that weren't over the top powerful.
I question why Bruce needs powers though. He already has the greatest power of all; DC favoritism. Batman is already an unstoppable force that nobody can compete with; he takes down kryptonians with spit, speed gods with glue, he's humiliated the Omega god with his bare hands, shattered the Source Wall, blown up the timestream, and makes everyone around him dumber just by proximity. Seems like he's plenty powerful already.