Add it yourself. Any game on Steam can be your rival, suggested or not.
- Open Rivals.
- Paste the game's Steam store link or app ID under Add a rival.
- Click Add rival.
Suggestions come from the Steam tags you share with other games. A game can be missing because its tags differ from yours, because its page is new and has few tags, or because your own page still has only a few tags. Fresh suggestions arrive every day as pages fill out.
If every slot is taken, remove a rival first or upgrade for more.
Tip: rivals you add yourself are kept active first if your plan ever changes.