Your idea would apply well to universities. As more students enroll, the universities could then display an icon, showing that they intend to expand, and the player could then designate some of the nearby area as an addition to the campus. The resulting new buildings could then benefit the university and the surroundings; for example, a fitness center improves health among students, a library allows students to graduate faster (effectively increasing capacity), a stadium attracts tourism. And the expansions done would reflect the surroundings; for example, the new buildings would be less likely to be dorms if students can easily get to the university from residential areas elsewhere in the city.

As far as other services, particularly transportation services, I'd like to see more modular construction instead. This will allow greater flexibility to the player with combinations such as those planned in the add-on and to expand such combinations as necessary to keep up with the needs of the city as well as any plans the player may have. Indeed, many of the assets available right now were created in part to overcome these limitations.

For example, instead of plopping down a subway station as is done now, with its one entrance and one platform parallel to the road, with modular subway stations the player can create multiple entrances and one or more platforms in any direction. The player can then add more entrances if necessary (for example, crowds of pedestrians are crossing the 6-lane road next to the station entrance, showing that access is needed on both sides) or more platforms (adding a new subway line to an existing station without changing the existing station).

Same goes for railway stations. Indeed, for railway stations it may be even more useful than for subways; the platforms may need to be built above grade, below grade, at grade, or some combination of these in varying directions depending on the planned railway connections, and platforms may need to be added as traffic increases. And when the subway is expanded into the area, a subway platform or station connection can be added directly to the railway station, leaving the rest of it unchanged.

Or for a non-transportation example, consider the schools. As the neighborhood goes up in level, the school can receive upgrades such as new classroom buildings, a gymnasium, a library, perhaps a school bus depot. Each of these upgrades can increase the capacity of the school, increase the value of the surrounding land, improve accessibility to the school, or otherwise improve its effect on the city.