Well since no one has posted this yet, I will.
We need a sequel for Sonic 4: Episode 2 in order to complete the Sonic 4 Episodic trilogy.
"What will Episode 3 consist of?" you might be asking.
Well, it's pretty apparent that we are going to have Knuckles as a playable character as it will follow the Classic/Sonic 4 formula of adding a new playable character for each new installment of a series of games. This game will probably draw inspiration from Sonic 3 & Knuckles where you could choose four different character setups: Sonic, Sonic & Tails, Tails, and Knuckles. What would be different is that there will be different paths/campaigns depending on the character you choose, taking alternate paths in the same levels and fighting slightly different bosses. The special stages could also be the same format as the special stages in Sonic 3 & Knuckles, which I believe are the best and most challenging in the series.
So yeah, let's make this happen Sonic Team.
And before I get a swarm of responds, I am aware that they are not working/making Episode 3, and probably will not revisit that possibility in the foreseeable future.