The ellipsis (...) as it is used nowadays has often struck me as being a sign of laziness... an unwillingness to finish a sentence. In other cases, it emphasizes the person's indifference and total lack of interest in the topic being discussed.
- Meh,... I guess so
- Yeah, ... whatever
In the OP's scenario, the message received:
might express the texter's indecisiveness or hesitancy, the number of dots acting as a visual indicator of their thinking in progress, in other words, the person is saying "I need more time to mull things over". Conversely, the three dots might represent the writer's decision not to express a direct opinion, otherwise known as hedging. The ellipsis could also be hinting a lukewarm disagreement, especially true if it is the only response given to a detailed proposal. If, however, the response was immediate it might be interpreted as a sign of politeness, the texter/sender acknowledging they have read the message and the ellipsis implies the promise of a more detailed and thoughtful reply in the not too distant future... but I wouldn't bet on it.