This is a very loaded question, I will try to answer.
First, yes, you can buy a new fork or a rear shock. The front fork replacement offers the maximum net benefit provided you buy a really good one. Any other part of your bike except the frame/motor/electricals can be easily replaced.
You cannot swap between Bosch and Yamaha motors, obviously.
To give more precise advice about what modifications make sense we would need to know your AllMtn model (like, is it a 5.0 or 8.0 or what). The current AllMtn 10.0 is already so well spec'd out that there is very little to change.