Until version 1.0, We’re focusing on the Bootstrap grid, to accelerate our path to launch as much as possible. However, over the next few updates, Bootstrap will be made into a more pluggable component, where it will only be loaded if bootstrap blocks are enabled.
At that point, we’ll start approaching building blocks for other grid systems and frameworks.
If you need to add support for unsemantic before that point, I’d recommend adding a blockade folder in your theme, and copying the “simple columns” block from the addons folder into it. You can then change the slugs being used throughout the plugin (to avoid conflicts), and modify plugin.js to use unsemantic classes, instead of Bootstrap. Lastly, you’d want to use the wp-blockade-available-addons
filter to ensure that the original simple columns addon isn’t loaded.