Right know we used the CRM link integration with contracts, but There is a lot of limitations in the intégration design and some buggy things. It would be greate if we can achieve this with the API.

Currently we can :  Create a counterPart and its contact, then Create the contract itself.

For me the next step and needs is : 

1 - Pre-Populate Contract
pre-populate custom field inside the contract with the api so my collab have just to read it and make correction if needed but the contract is ready to go.

For this my suggestion is :
A)  to make an api call to get the list of custom variable inside the contract response by id; key and type this can be attached to the contract definition if possible or separate.

B) to make another api call to populate a contract custom field and save it or attached this fonctionality directly to the create contract call.

2) Modify by api the selected variation of CLAUSES, 
      
      Idem as the 1rst one : Get list of clause variation available and select the actives one on the contract.  

Thanks for reading me !