Here's an fun minicombo i use occasionally to get a solid multicolor manabase.
Gaea's Balance +
Planar BirthNot only does it get you all 5 colors, it gives you 1 more card advantage than Ancestral Recall with the same net cost

.
I actually seen a Sliver deck with the manabase of 4 CoB, 4 Thran Quarry, 4 Gemstone Mine, 4 Archeological Dig, 4 Reflecting Pool, 2 Undiscovered Paradise, and 2 Grand Coliseum. To make a long story short, it sucked not because the manabase was weak (you don't see that many wasteland in casual play), but because you really needed to punch out a lot of slivers to present a threat.
Since you already have a Sliver Toolbox set up, why don't you diversify your slivers? A lot of the older slivers are amazing under certain circumstances, such as
Acidic Sliver and
Victual Sliver. I would suggest running them as 1-ofs.
Horned, Plated, and Blade Slivers always felt really weak. Slivers generally are exceptionally weak with amazing abilities granted by the Legions slivers, so why don't you make them scary disablers instead of mediocre attackers? I find a combination of Hunter, Toxin, and Synapse Sliver to be better. Plus Shifting Sliver makes Horned look bad.
If you do choose to use the combo i listed above, why don't you consider
Spectral Sliver? Instead of giving minor boosts here and there via Plated and Hunter, Spectral makes them into mini Nantuko Shades.