Amy for any real-time action game I'd probably use Node with TCP packets, SQL and even Firebase aren't fast enough to keep up with real-time movement and combat, but TCP packets these days are acceptable (Minecraft, World of Warcraft) so that would save having to write a full packet system.
If you didn't go for an ABS, then Firebase database alone would be sufficient for tracking player movement - but it's not suitable for combat that relies on real-time movement.
I would love to build an online system for MV with Firebase. Wish I had more time than just weekends to work on cool experimental MV projects. I wonder how much people would pay for an MV online development kit.
Weekend RPG Maker MV plugin writing;
It's a 3D battle system.
Because software rendering is something MV supports, I'm writing a software and hardware renderer for the 3D backgrounds. I'll have to write an auto-tessellation algorithm to balance performance against visuals dynamically.
Sucks because I'm pretty sure only 0.01% of MV users depend on the software renderer, but MV supports it so Plugin writers need to as well.