An installment of a large series that started its life as a Spin-Off or an Expansion Pack, but eventually became the next mainline entry in it, often due to scope creep or to Executive Meddling (when the execs demand a sequel shipped now, rather than when it's ready).
See also More Popular Spin-Off, which may be the cause or the result of this.
- Dragon Age II wasn't originally intended as a sequel to Dragon Age: Origins — instead, it was conceived as Dragon Age: Exodus, a Spin-Off game with a much smaller scope. However, after Origins became a massive success, publisher Electronic Arts demanded from BioWare a fully-fledged sequel within less than 16 months, forcing them to repurpose Exodus, with polarizing results. Downplayed with Dragon Age: Inquisition, which was conceived as a mainline entry from the get-go but had to retroactively absorb much of the plot from the DA2 expansion Exalted March, which was planned but scrapped after its main game underperformed in sales.
- In 2012, Volition was working on new DLC for Saints Row: The Third, titled Enter the Dominatrix, as well as the next mainline entry, Saints Row: Part Four, which was to be set in a whole new city. However, its parent company THQ experienced financial troubles and told Volition to abandon Part Four and to turn EtD into the next main installment, released in 2014 as Saints Row IV. The game proved even more controversial than The Third, mainly because it recycled the latter's world map almost without alterations (for the first time in the series) and took a sharp turn away from urban crime sandboxing into quasi-Super Hero territory. Later DLC for Saints Row IV, also named Enter the Dominatrix, explores the plot changes between the original DLC and IV, such as Zinyak being changed from a Disc-One Final Boss to the Big Bad and others.
- The Persona series is a downplayed example: having started out as a spin-off of the Shin Megami Tensei series, it has reached equal heights (especially in the West) without fully eclipsing the original games, which are still going strong.
- Final Fantasy XV began life as Spin-Off game under the Fabula Nova Crystallis banner, originally named Final Fantasy Versus XIII and sharing the mythos with Final Fantasy XIII. However, Versus XIII went through a lot of Development Hell. The rebooting of the project and turning into a mainline installment was basically an attempt to salvage it.
- Resident Evil 3: Nemesis for the original PlayStation wasn't originally conceived as the third mainline Resident Evil installment, but a side game. The third installment was originally supposed to be the one in the works for the PlayStation 2. However, development of the PS2 game ran into issues due to the new hardware, which likely would've resulted in a longer wait between installments. As a result, Capcom made the side game into the official third installment. In turn, the PS2 title became the fourth numbered installment: Resident Evil 4.
- Fallout 3 has a side quest in which you help (or betray) an android from the Commonwealth escaping from the evil group of scientists known as the Institute with the help of a group called the Railroad. Fallout 4 takes place in the Commonwealth, features the Institute as major villains (unless you join them), has the Railroad as a major joinable faction, and prominently features synths in the plot.