It has been long rumored that Rare has plans to revisit older IP and that Microsoft has given them more freedom, leading to Sea of Thieves being announced and impressing many. This extends to more titles coming from the studio, as a rumor is coming out that Rare is working on two new projects.

Apparently Rare is working on two IP’s they aren’t ready to discuss yet, with two smaller teams within Rare. Possibly shown at Gamescom. (not counting Sea of Thieves, which in itself looks great)
Source comes from The Inner Circle Podcast of TICGN.com, from what I know the man in the middle (Kor x Kal El I think he goes by) is someone who has hosted interviews/talked personally with Xbox executives such as Phil Spencer and Mike Ybarra and has regular talks with other notable figures at the Xbox division so there’s a bit of credibility, but he doesnt say who he got this information from (apart from ‘some people at E3’) so take it with buckets/truckloads/freighters of salt. “Two extra IP’s” is also vague as it could mean existing IP’s, new IP’s or both.
This would mean three different teams at Rare would be working on Sea of Thieves, Game X and Game Y simultaneously.

 
Taken from NeoGaf and its source being The Inner Circle Podcast, its clear that Rare has plans after Sea of Thieves releases. The likely situation based off my opinion could be one of the following.

• Rare works on Battletoads & a New IP
• Rare works on a new IP & a revivial of an older one (like Banjo or Perfect Dark)
• Rare works on Battletoads and co-develops a new title with another studio (not unlike them working with the team behind Shovel Knight to get the Battletoads in the game or how they let Double Helix/Iron Galaxy work with the Killer Instinct IP)

Looking forward to seeing what this rumor leads to and hopefully we get an announcement on future Rare titles in the future.