The proposed work is about influence on gamification in Customer brand engagement but unable to decide that in current scenario which category is more impactful.
Gamification in online brand communities and in brand app experience are widely discussed topics nowadays, but they have received very little scholarly attention. So, you can select these categories to make a significant contribution in the literature
There would be multiple scenarios that would be plausible. I do recommend that you take the "practice" as orientation and justification for your study. Hence, have a look at practical examples and see where and how gamification is used here to engage consumers with a brand. Then copy and pre-test different scenarios before you draw a decision for your own study.
Would you like to share some more details about your research idea beyond the "influence on gamification in Customer brand engagement" to help us give you some useful suggestions? Basically, you first need to develop your research objectives based on a thorough review of the literature and only then think about the methodology and context of your study!