My ds is obsessed with firetrucks/firefighters. It has been like this for probably about 2 years now, just getting stronger and stronger. At this point he pretty much exclusively plays firefighter. He turns everything into a firefighting game somehow. He talks about firetrucks all day long. He makes siren noises the whole time we drive somewhere.
His birthday was in November and everyone knows he loves firetrucks. He received 6 firetrucks and a fireman dress up set. This is in addition to the firetruck & firehouse he already had, plus his fireman rain coat and rain boots.
For Christmas we really encouraged him to branch out and look around for some new ideas and likes. Among other things, he received a pirate ship and some astronaut/space figures. After the first day the pirate ship sat totally unused. Until...the police arrested the pirates, the firemen took over the ship and turned it into a fireboat. Now it gets used every day--as a fireboat.
Once ds realized all his unused toys could be repurposed into firefighter toys, that is what he did. His astronauts now drive firetrucks too. His cardboard playhouse is a firetruck. When he dresses himself he only picks firetruck shirts. His Duplo tanker truck is a firetruck. The Duplo police station is a firehouse. He only draws pictures of firetrucks. He literally talks about firetrucks all.day.long. It never stops. Every conversation is about firemen somehow. He takes dd's dollhouse and pretends it is on fire and saves her dolls with his trucks. Library books are all about firetrucks or firemen, always.