They can change many things since re 3 take place in the street of raccon city while re 2 take place just inside the police station...
They could make it a bit more opened, whit a big map, but not as big as an open world, just 4 time bigger than Re 2 and put more secret in this map... Put more zombie that you have to dodge like horde of zombies and some survival gameplay but just a bit not too excessive, and than there is nemesis, it would bee like Mr x but he will fallow you into the entire city whit weapon like the rocket launcher, and you have to hide into building, like house, shop, restaurant and you can use object, in this building to knock him out for some times, you can also make trap to use against nemesis and other enemies...
They can also add some people around the city that you need to help, maybe you see real people that are hiding into house and you help them by defiting zombies in the area or by taking supplies like wather and food to them...
They can do many things whit re 3