Nothing wrong with rolling your own comms protocols. Depending on what your trying to achieve, and how complex your requirements are, some things you might want to think about up front could be, in no particular order:
Will it be used to communicate one to one (point to point), one to many or many to many nodes (bus or network type environment)?
How far in distance terms must the signal reach before degrading beyond being reliably received?
Over what medium will the communication take place? Wires or Wireless?
What type of Cable or Radio module will be used?
How much data, in bit terms, is required to be sent and received at any one time, or is this a continuous stream of data bits?
How fast must the data be transmitted/ received etc?
Will error detection and data recovery be required?
If a bus/ network type environment, how will you detect and mitigate possible data collisions?
This list is by no means exhaustive, and depending on your goals, may totally be overkill, however, whatever else you do, try to keep your whole vision as generic as possible, you will want something that can be sent over any medium at reasonable speed and be robust enough to recover from data errors, all with minimal changes that account for the medium being used. Most serial protocols go hand in hand with their hardware or physical counterparts, so the resulting protocol is usually a mixture of two things, control bits and data bits