I would love to see Galerider Sliver (and certainly other missing cards) added to Pioneer on MTG Arena. We have Galerider in paper but would love the opportunity to test with it on Arena as well. It's a major missing piece for anyone trying to make Slivers work in Pioneer on Arena.

Edit: Warping Wail would be great to see as well. Slivers in Pioneer would use many rainbow lands (for creatures) and a spell like Warping Wail would be a great tool to have since the rainbow lands would not produce colors for noncreature spells (exception being Mana Confluence).