The best alternative in my opinion is ulauncher.
It's discussed to include it as a standard app in debian and maybe ubuntu will follow.
The ulauncher homepage gives many plenty of extension and also how to build your own extension.
It is highly configurable and works with plasma (baloo, ...) and gnome.
For installing follow the description on ulauncher download section
, where a deb or ppa installation links are given.
For the ppa use:
sudo add-apt-repository ppa:agornostal/ulauncher
sudo apt update
sudo apt install ulauncher
And to run it as a service using systemctl you may copy the following script into your service directory:
Description=Linux Application Launcher
Now you can start or enable ulauncher as service.