Top 10 players Daniil Medvedev and Stefanos Tsitsipas have confirmed they will play at Queen's in June.
Spain's defending champion Feliciano Lopez will return, while organisers hope Britain's Andy Murray will recover from a pelvic injury in time to play.
Former world number one Murray, a five-time champion, has not played since November's Davis Cup finals.
"Daniil and Stefanos have shown they can beat anyone in the world," said tournament director Stephen Farrow.
"Feliciano's win last year was incredibly emotional and uplifting, and Andy is our greatest champion."
Three-time Grand Slam champion Murray was forced to pull out of January's Australian Open because of the pelvic injury.
The 32-year-old said last month he was not putting a time frame on his recovery as "bone bruising" is taking longer to heal than expected.
"We know Andy is still working his way back to fitness, but we also know that he wants to play, so fingers crossed that he will be able to," added Farrow.
Russia's Medvedev, 24, is fifth in world rankings after reaching the fourth round in Melbourne, while Greek 21-year-old Tsitsipas, who won the Open 13 tournament in Marseille earlier in February, is sixth.
The main grass-court event will run from 15 to 21 June and be live on the BBC.