Generate new sentences based on keywords
This is area of NLG . You can use template based text generation techniques, wherein you have defined structure of output text and fill in required blank areas based on keywords. This technique is used in reports generation. An example is narrative science company. 

Other approach can be to use OpenAI GPT . Example is Generate Text using OpenAIGPT2 in Python
 . You may have to tweak the code as per your requirement.  

Paraphrasing can be another technique. An example of paraphrasing is - https://github.com/vsuthichai/paraphraser