There are two ways you can go with this. Either just take the plunge, do it, and deal with the potential consequences (a temporary dip in viewership, some unfollows) or gradually merge it in.
A general tip given is to have set days for what type of game you play. Let’s say you are known as a Minecraft streamer. Mon-Thu you play Minecraft and then on Friday you introduce a random variety game. From here, you start to get a new set of viewers dropping by. They’ll start to spread across to your other streams and vice versa. You can later assess whether to alter the ratio further or whether you’ve found a healthy balance for your channel.