CONSIDER ADDING:
Descendants' Path
Semblance Anvil
Thawing Glaciers
Syphon Sliver
Sliver Overlord

I had Watcher Sliver x2 but replaced them with Pulmonic Sliver x2 because I discovered that a win condition is to have flyers.  Without flying, it won't compete.  Also, the ability to choose to place the killed slivers on my library allowed me to setup my draw step next turn, and also stack my Call to the Kindred.

I had Sedge Sliver x2 but realized only 4 lands produce black mana that can use it.  Usually I had a Swamp, when this sliver was in play, however I didn't have enough black mana to cast my spells AND regenerate slivers.  Therefore, it was essentially a +1/+1 2/2 which costs 3.  It was replaced with Necrotic Sliver x2 for removal of artifacts, enchantments, Planeswalkers, and any big/pesky creatures; it can be paid with colorless mana.  Necrotic Sliver x2 was chosen over Harmonic Sliver x2 because it is a 2/2 as opposed to a 1/1 and it wasn't limited to just artifacts & enchantments, and I could use the ability on my opponent's turn.