Cryptolith Rite is a pretty solid way to shore up your mana, basically copy 3 of Manaweft Sliver/Gemhide Sliver.

In the same vein, Mirari's Wake really helps to ramp up, both your mana base and your creatures.

Herald's Horn lowers the cost of your Slivers and provides repeatable draw. Very useful.

Phyrexian Reclamation is a repeatable way to return cards from your graveyard.

Shared Animosity doesn't affect your opponents creatures, like Coat of Arms does.

Spirit of Resistance works great with 5c commanders. In particular, it's an amazing combo with Sliver Hivelord.

Paradox Engine gives you essentially an unlimited combo with Sliver Overlord, letting you put all of the Slivers in your entire library into play if you've got Manaweft Sliver/Gemhide Sliver/Cryptolith Rite in play. Even if you don't have the combo pieces in play, it can provide a ton of value.

Distant Melody and Wheel of Fortune are such good draw cards that they're hard to pass up.

I too play a Sliver EDH deck, so these are some suggestions that I've learned from experience can be rather powerful.

As someone who currently building a Sliver Queen deck, and new to magic, this is exactly what I needed to see. My deck has mana echoes but I knew I was missing something, so many more opportunities to go off relying on ONE CARD. Our mana base(though I have yet to update mine) is exactly the same so I know i’m headed in the right direction with that, and I have all the other pieces in my trade binder but I just never thought to do something this extravagant. This deck is simply amazing and I will definitely be updating my deck because of it. +1 to you good sir!!