One of the topics that is of growing interest is to use a hypervisor on an applications processor alongside a TrustZone based TEE. This new white paper from Mentor give a great introduction to the topic.
I would add that the growing popularity of ARM Trusted Firmware
makes the integration of these systems much easier than it used to be. On ARMv8-A (64/32-bit architecture) based platforms we have a new exception level (EL3) which is typically used for Trusted Boot and a small run-time doing the world switch, PSCI, interrupt routing etc. ARM Trusted Firmware provides a reference implementation for this EL3 code and has been ported to many platforms including our own Juno development board.