Ask HN: Suggest an advanced 2D engine for an Open-Source Hobby MMO development

What is the most advanced (both feature wise and ease of use wise) 2D engine out there that can help me reach the goal of developing a basic 2D MMO quickly. I would like to keep initial development Open Source and then maybe fork it.

The engine preferably should have a networking API that lets you go to thousands persistent users scale easily. This will be a huge plus.

Also advanced things like pixel based collision detection, shaders and scripting etc would also be welcome.

BUt mostly engine should be well designed so I don't hit a bottleneck scratching my head what to do and how to do it.

I have MMO development experience in Crystal Space engine (Planeshit.it was MMO).