Network slicing is a relatively recent paradigm that has become a subject of intensive research. Most of the existing approaches follow the ETSI NFV MANO model with some extensions related to multi-domain slicing, slice selection, etc. The framework is a part of the standalone variant of the 5G network standardized by 3GPP. However, the current implementation of network slicing in 5G has several limitations, especially regarding management and orchestration: The isolation of slice management is not appropriately addressed, the slice tenant management capabilities are limited, and the management and orchestration centralization raises serious scalability and reliability issues. Moreover, the slice-level operations are not well-separated from other processes due to a lack of proper separation of concerns. As a result, the overall network slicing architecture has complex interactions with many components of the 5G network. In this paper, we describe a new framework that uses an approach to sustain self-managed and self-orchestrated slices. To that end, we propose a modular architecture in which slices have embedded (in-slice) management and orchestration support, and multi-domain slices rely on multiple slice-agnostic orchestrators. The framework enables the composition of new slices as a combination of single domain-slices in a relatively easy, technology-independent manner.