Major modifications could be necessary to ensure a safe aircraft which could require a new type certificate. Boeing are caught between doing enough to restore safety but not so much that the MAX loses its Grandfather rights from the original design. Basically this is what they tried to do with MCAS and it didn’t work.
An interim measure could be to try and fit a FBW flight control system to the MAX and certify it as a new type which would be very expensive and time consuming but less so than a clean sheet design.
Unfortunately Boeing could be caught up in the trade dispute with China and unless the USA lays off on Huawei, the Chinese could refuse to accept any fix, declare the MAX unsafe and ban it from operating in their airspace. Their own narrow body is only a couple of years away from entering service and whilst not quite matching the A320/B737 its a credible enough alternative. Performance improvements will undoubtedly follow over the years as they have for most aircraft types.