Ask HN: HN Style Aggregation Legalities

Suppose a site like HN existed, except that when articles/links are submitted their contents are extracted and the content (while still properly attributed) is displayed on the aggregation site itself. Are there legal boundaries being crossed here or is this considered fair use?

Context: I am thinking about creating a browser based niche reader app that would allow users to stay on one site while viewing content from multiple sites. The content would automatically be scraped from RSS feeds. If the RSS feed does not provide full content, the scraper will get the content from the full URL.

Thanks in advance.