You could achieve this with a combination of Butler (Trello's automation tool) and the Custom Fields Power-Up.
Basically you'd set up a Text field to display on the front of the Card called, for example, "Last Comment".
The Butler rule to then show the last comment could look like this:
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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