What is an alternative ending for “Bliss” that portrays what happens next?
To answer this question you need to think of how the story operates and in particular the themes that it presents. One of the more subtle themes of this excellent tale is that of change and transformation. Throughout the story we witness Bertha's increased awareness of her sexual attraction to her husband. This is of course symbolised by the rather fecund pear tree that is in full blossom--a clear symbol of her sexual awakening. However, at the end of the story, after she has witnessed her husband kissing Miss Fulton, this transformation is brought to a complete halt, signalled by the fact that the pear tree remains the same and has not changed further:
But the pear tree was as lovely as ever and as full of flower and as still.
There is no sense of continued change and transformation, and this is something that any alternative ending would have to capture. You might, for example, detail the conversation that Bertha has with her husband. I personally think that any such conversation would not actually make any reference whatsoever to Bertha's knowledge about his adultery. Note that the pear tree is described as being "still." This is something that could be captured through Bertha's secret awareness of her husband's affair and how it leaves her trapped and unable to express her own sexual feelings: a very tragic ending to a story that is, at least in part, about self-awakening and self-awareness.