The phrase "new mainline" refers to a fresh or updated route or path that has become the central or primary option for something. It indicates a change or shift in direction towards a different course. Full definition
The accompanying press release does clarify that this will be a «new mainline game» for the series, and it's the first developed for a home console since Fire Emblem: Radiant Dawn on the Wii in 2007. (dualshockers.com)
Whether you get giddy on the sight of Disney characters or have spent the final 16 years piecing collectively each intricate element of its spiraling plot, a brand new mainline entry within the sequence brings a sure degree of fervor. (skilledgeekz.com)
Last week, Nintendo made two major announcements during its Nintendo Spotlight presentation: Metroid Prime 4 and a brand new mainline Pokémon game are both coming to the Nintendo Switch. (perfectly-nintendo.com)