We will find out soon because Arlovski is the #4 ranked heavyweight 
I think that Mir has the skillset to beat anyone in the heavyweight division. He has the power to KO anyone at any given moment, and has one of the best ground games in the UFC. Here are the current top 5 fighters in the heavyweight division:
Champion: Werdum
#1. Cain Velasquez
#2. Junior Dos Santos
#3. Stipe Miocic
#4. Andrei Arlovski
#5. Travis Browne
Tell me which of these fighters do you think is invincible?
None of them are invincible but they are all moe than good enough to beat a post-prime Frank Mir. His striking is good enough to beat mediocre/mid card strikers with no plan B but his ground game can beat anyone. However:
Werdum - comparable size, better BJJ & also has incredibly improved his striking. He takes this by stoppage IMO.
Cain - dominates every area, would overwhelm Frank and pummel him like Lesnar did. Easy stoppage.
JDS - been there, done it. Would do it every single time they match up.
Stipe - another of comparable size, would be the closest matchup of the top 5 IMO. Still, he takes it by comfy decision or TKO.
Arlovski - better striker by light years, good sambo, would spark Mir out cold in any exchange as long as he didnt expose his chin.
Travis - too big, too powerful, Mir would have to get him down to take control. Another big KO loss.
These are just my opinions, Frank may very well come out and smash Andrei which would indeed put him in a title eliminator. I would be shocked to see him compete for or even get near a title shot again though.