How to develop a useful TA?

[targetnull]

Hi Jens,
I have compiled your Hello_world TA and it worked well on FVP. Now I want to develop some useful TAs such as critical IO control and syscall hook, the problem is that these things seem to have nothing to do with TEE or TrustZone, it can be done in normal Linux system without communication with Trusted OS.

Could you pleaze give me some suggestions on how to develop a useful TA? I think the most important issue is to define what kind of service should be regarded as a secure service and thus be put in the secure world. There is currently not much documents to describe it.
Thanks in advance.