I have seen a few different supplements that combine spells in various ways, but I have never seen the idea of a sequence of effects where one ends and the next takes over. Here are a few examples using AD&D spells:

Congealed Flame is a 5th level arcane spell that initially creates smoke (Fog Cloud) that will collapse into a sticky mess (Web) that ignites in the last round (Fire Trap) that destroys the pseudo silk.

Swat Foe is a 3rd level arcane spell that weakens a target (Ray of Enfeeblement), then drives them off (Scare) and finally destroys their equipment (Shatter). As the enfeeblement ends, those that do fail their save and run move as fast they they can to escape the caster.

Stone to Ice is a 6th level arcane spell that creates mass of rock (Wall of Stone) that transforms into an ice crystal (Wall of Ice) that in the final round explodes (Ice Storm).

Insect Army is a 5th level divine spell that first creates a habitat for the insects (Plant Growth) that will shrivel and spit out one or more huge insects (Giant Insect). Instead of shrinking back to normal size, the giant insects will break up into smaller version of themselves (Summon Swarm) that will obey the caster and then disperse into the surrounding environment.

This also can be used for spells that have a decaying effect, such as a version of Repulsion that gives one last Push in the last round.

In any case, the duration should be equal to the spell with the longest duration, excluding permanent or instantaneous, and a successful saving throw ends the sequence spell at that point. For specifics (level, area of effect, saving throw type) that is up to the Dungeon Master.

If you do use this idea, please tell me how it went.