This is a build I put together to have some fun with summoning before the game comes to an end.

The idea is to get as many summoned creatures into play at one time that I can. Although a balance between number of creatures and other stats (usually crit chance) had to be struck.

Unique's, Artifacts, Medallion and Insignia were all chosen with that in mind.

I went with Doom for this experiment as the Eternal Guardians and Deploy Doombots trait allow for deploying 6 summons with a single power.

Rotation is fairly simple:   to get the summons into play,  and  to apply DoT and Vulnerability. And the your choice of  or . Pop the signature,  as available.

 was used while leveling as resource management was an issue. However, the You Cannot Hide trait makes  a free power so resource is not usually an issue. So may replace with  or 

Magik is the only team-up that I'm aware of that provides additional summons, so no other choice for this build.

The build has the potential to put a total of 15+ summons into play at one time (Duration/Cooldown)

1 Team-Up (Magik)
4 Minor Limbo Demons (from Magik) (8/30)
1 Major Limbo Demon (from Magik) (12/30)
1+ Random Being (A.R.M.O.R. Alternate Reality artifact) (21/??)
1 Wizard (Wizard's artifact) (10/30)
1 Doomdrone (Mask of Doom artifact) (60/10)
1 Colossus (Sinister Medallion) (35/60)
1 Wolverine (Sinister Medallion) (35/60)
1 Cyclops (Sinister Medallion) (35/60)
1 Herbie (Herbie Insignia) 
1 AIM Headwalker (Doomsday Chair unique) (33/60)
1 Sentinel (Omega Sentinel unique) (30/60)

Some other items considered 

Man-Ape medallion - adds crit/brut rating and summon bonuses, but no summon
Mole Man medallion - summons a moloid leaper (30/60)
Nick Fury Insignia - summons a SHIELD Tactical Agent (30/30). Don't have one would probably replace the HERBIE insignia if I did
Artifacts
Symbiotic Infestation - summons 2 symbiotes (15/30). Other affixes don't work well with Doom
AIM Robot Dispenser - summons 2 AIM Death Machines (60/10). No cosmic version
Advanced Wungadore Clay - summons 2 raptors (60/10). No cosmic version
Mole Man's Staff - summon Moloid Leaper (30/60). No cosmic version.
SHIELD Identification badge - summon SHIELD agent (30/60)
Slot 1
Ruby Topped Cane - summon 1 Hammerhead Capo (30/60)
Slot 2
Eidolon Warwear - summon 3 War Drongs (30/??)
Slab of Subterranea - summon Moloid (30/60)
Symbiote Infection - summon up to 5 symbiotes (30/??)
Slot 3
Girdle of Frost Giant Strength - summon Frost Giant (30/60)
SHIELD Reconnaissance Belt - summon SHIELD Agent (20/60)
Slot 4
I found nothing useful for this build in this slot!
Slot 5
Trask  Commander Helm - summon Sentinel (30/60)
Mask of Algrim - summon Frost Giant (30/60)
Octobot Controller - summon buffs