Here are some useful tools and strategies to complement App Engine and improve the visibility and ranking of your mobile app on the Play Store and App Store:

App Store Optimization (ASO) Tools:

Sensor Tower: Provides insights into keywords, app performance, and competitor analysis.
App Annie: Offers app market data, analytics, and insights to optimize your app store presence.

Keyword Research Tools:

Google Keyword Planner: Helps identify relevant keywords for your app.
Ahrefs: Useful for researching and analyzing keywords to understand search volume and competition.

Performance Monitoring and Analytics:

Google Analytics for Firebase: Provides in-depth analytics on user behavior, engagement, and conversion tracking.
Crashlytics: Identifies and analyzes app crashes to improve stability.

User Feedback and Review Management:

Apptentive: Engage with users through feedback, surveys, and in-app communication.
Appbot: Monitors app store reviews, analyzes sentiment, and helps manage responses.

App Marketing and Promotion:

Facebook Ads and Google Ads: Run targeted advertising campaigns to increase app installs.
App Samurai: A mobile advertising platform that helps with user acquisition and app promotion.

User Engagement and Retention:

OneSignal: Implement push notifications to re-engage users.
CleverTap: Offers personalized messaging and analytics to boost user retention.

A/B Testing and Optimization:

Split.io: Enables A/B testing for app features and functionalities.
Optimizely: A/B testing and experimentation platform for optimizing user experience.

App Security:

OWASP Mobile Security Testing Guide: A guide for securing mobile apps.
Google Play Console: Monitor security issues and receive alerts for potential vulnerabilities.

Social Media and Community Building:

Buffer or Hootsuite: Schedule and manage social media posts to promote your app.
Reddit and other forums: Engage with communities related to your app’s niche for organic promotion.

Localization Tools:

OneSky: Streamlines app localization and translation.
POEditor: Collaborative translation platform for multiple languages.

Remember that the effectiveness of these tools depends on your specific goals, target audience, and the nature of your mobile app. Regularly analyze the data, gather feedback, and iterate on your strategies to continually improve your app’s performance and visibility in app stores.