Sonic 2017's Official Title is Sonic Forces, Sonic Mania Delayed to Summer

SEGA announced a pair of updates regarding the two Sonic the Hedgehog games the publisher has in development.
The first gameplay of modern Sonic in Sonic Forces is coming soon.

Project Sonic Is Now Sonic Forces

Sonic Fans Club

Sonic Mania Delayed; Project Sonic 2017 Gets A Name - GameSpot

Sonic Mania delayed to summer 2017 [Update] - Gematsu

Sega came up with a way to get fans excited about Sonic the Hedgehog again

2017 – A Year In Supersonic Review

SONIC MANIA Delayed Until Summer 2017 — GameTyrant

Sonic Mania Delayed To Summer - Fans Told It Takes Time

Sonic Mania Preview - Knuckles Returns To Flying Battery Zone In Newest Sonic Mania Trailer - Game Informer