So your main complaint about Padrino is Padrino::Application as an extension to Sinatra? Because in every other regard, Padrino (padrino-core) is mostly a convention on how to load and organize code. I use it as a raw library on a regular basis.

I admit that we are a bit behind in regards of apps-as-libraries, but I would prefer to handle that in standardized ways (using gems as distribution format and copying assets using an asset compiler instead of an installation routine).

Also, Padrino encourages not developing any plugins for it - raw Sinatra plugins are very much preferred, but are not compatible to your format.