Gardener:
In the Infinity Gauntlet comics, the Elders of the Universe played an important role. Mar el Cinematic Universe already has The Collector in its Roster and Thor:Ragnarok will introduce the Granmaster. Dinklage could definitely go up for the part of the Elder called The Gardener who is a pacifist when compared to the other Elders. The gardener,like his name suggests loves his plants.
Ebony Maw:
Ebony Maw is the manipulative but highly intelligent member of the Black Order. Given that Dinklage stars in a show about political intrigue in his day job, Game Of Thrones, it won’t be much of a stretch for the actor to sink his teeth into this role.The Black Order serve Thanos.
Mephisto:
Mephisto is Marvel Universe’s version of The Devil. He was a consigliere/counsel for Thanos in Infinity Gauntlet. He is most associated with the character of Doctor Strange and Spider-Man, so his role would be a link to both heroes. If Mephisto does make his debut, Dinklage would have to a voiceover actor like Josh Brolin, who voices Thanos.