Being Nim-based means the result can be compiled to C or JavaScript, so it can run at native speed or in the browser.
- HaxeFlixel (https://haxeflixel.com/) (Extremely portable)
- Phaser (https://phaser.io/) (HTML5 framework)
- Nico (https://github.com/ftsf/nico) (The PICO-8 API in Nim)
- Pixel Vision 8 (https://pixelvision8.github.io/PixelVision8Website/) (Another fantasy console)
https://liko-12.github.io/ (lua, open source, I haven't used this much, but it seems nice)
https://github.com/ftsf/nico (nim, open source. Not quite the same as the others, there's no IDE, etc. But it's a library with an API similar to PICO)
It's got similar functions to PICO-8, but instead of lua you write it in Nim, and instead of an integrated editor, you use whatever editor you want.
Perhaps most of all though, the author also wrote some pretty cool games with it, which are also open source, so it's proven tech and you can learn from them.
Also for linear algebra and machine learning they're a couple of good libraries:
1) Arraymancer https://github.com/mratsim/Arraymancer
2) Neo https://github.com/unicredit/neo
3) my own :) not meant for when performance is required. https://github.com/planetis-m/manu