Yes, Yidris is four colors and that can make casting him difficult. and yes, we had a commander like this before with Maelstrom Wanderer, except that you got immediate dividends when you cast him. Cascade is a crazy ability that essentially casts you free spells. Having to deal combat damage with Yidris first, then having to cast more spells to get cascade seems like a bit too much work. But Yidris does have trample, so it's not all that hard for a 5/4 to get through for at least 1 damage. So is waiting to cast spells during the second main phase is worth the payoff?
Whenever you cast a spell with Cascade, it allows you to reveal cards from the top of your deck until you reveal one with a lower converted mana cost. This gives you a free spell, essentially. Maelstrom Wanderer allows you to Cascade twice whenever he is cast. This means you got two free spells. But Yidris takes that a step further, giving you every spell you cast from your hand Cascade. It doesn't give them extra chances to Cascade if they already had it, but getting two spells for the price of one is already silly (see: Bloodbraid Elf). And yes, you can play the Wanderer in this deck.
So what would a Yidris deck look like? It would probably look like a Maelstrom Wanderer deck except with Black cards included. Also, because your Commander's ability is based around combat, you'll want enablers to give you additional combat phases and main phases. While the combat phases are redundant, getting an additional main phase is important, too,especially if you get creatures with haste to attack with. Aggravated Assault, Fury of the Horde, Relentless Assault, and Seize the Day are the perfect cards for this scenario. Plus, you get to Cascade into as much as a 6-drop with Fury of the Horde and even a 3-drop with Relentless Assault or Seize the Day. Aggravated Assault is only 3 to cast, so you'll probably Cascade into it on a regular basis. Seize the Day has Flashback, too. You'll always want to Cascade into these since you'll get to burn through your deck a lot faster.
The rest of the deck will look a lot like your average Maelstrom Wanderer Commander deck, except that you get to include Black removal and other cool stuff, too. Extra turn spells, big splashy creatures and card filtering are the norm for Cascade decks. (You can even play Temporal Extortion!) When some optimal builds are finally discovered, Yidris' true power will become a lot more clear. I don't mind playing Bituminous Blast in a deck like this, that's for sure!