The best open source alternative to Mixcraft is LMMS, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Mixcraft and four of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Mixcraft are Bosca Ceoil (Free, Open Source), CrossBrowdy (Free, Open Source) and MusicBoard (Free, Open Source).
Bosca Ceoil is all about looping, and designed that way at its heart. Songs are built up from lots of tiny 16 note patterns – the intended workflow is that you loop over these single patterns until you have something you like, then you start making variations.