Mycosynth Lattice is in the maybeboard, but I suggest acquiring it as soon as possible. This allows you to bypass spending the mana on Memnarch's first ability, and of course leads to lots of other artifact shenanigans.

Grand Architect is also a great card to to consider; he ramps your mana, is a lord, and is a potential combo piece.

Thran Dynamo and Gilded Lotus are two powerful mana rocks that help ease the costs of Memnarch's abilities.

In addition to being a strong boardwipe artifact, Nevinyrral's Disk can be used as land removal as well by targeting problematic lands with Memnarch.

Blue Sun's Zenith is an excellent card draw tool for when you have lots of extra mana to sink.

It is a sin to not run Solemn Simulacrum, especially in an artifact-based EDH deck that needs ramp to play its commander.

Phhhh Rakdos, Lord of Riots Is easy and fun to build around. 10 or so early damage Guaranteeing cards that cost less than 4 but have abilities that burn without using mana, a bit of color fixing and you are likely to drop him turn 4. Make sure that he can hit turn 5 Then drop 2 or 3 large creatures from your hand for next to nothing. Comrad Rakdos tends to end games before turn 7 Unless you meant Rakdos the Defiler In witch case you just need Conspiracy or just demon tribal.