Two decks with every King stripped out feed three kinds of foundations, aces, evens, and odds, for a medium-difficulty game with a 65% win rate.

Carousel Solitaire is a two-deck game built around three different kinds of foundations: one for Aces, eight for evens, and eight for odds. The two decks here skip every King, so the evens foundations top out at Queen and the odds top out at Jack.
Two decks feed three different foundation types here, which keeps you juggling enough that this lands as medium difficulty — you'll win about 65% of the time, with skill and luck both playing a real part.
Other games in a similar spirit include Great Wheel, Greater Wheel, and Mount Olympus Solitaire.
The classic Spider Solitaire sits a click away, for when you want fewer foundations and more tableau to dig through.
If you run into anything odd or have an idea that would make the game better, please contact me.
Enjoy playing!
1 ace foundation: Built with just Aces, alternating colors, until all eight Aces (from both decks) are in the pile.
8 evens foundations: Start with Twos and build up by twos in suit (Two, Four, Six, Eight, Ten, Queen).
8 odds foundations: Start with Threes and build up by twos in suit (Three, Five, Seven, Nine, Jack).
8 tableau piles: Build down, same suit. Only one card can move at a time. Each pile starts with four cards, face-up. Empty spaces are refilled with four cards from the waste, or from the stock if the waste is empty.
Stock: Click to turn over one card to the waste. There are no redeals.
Waste: The top card is available for the foundations.
There are three kinds of foundation piles.
The Ace Foundation
One pile. Any ace can start it, and cards build in an alternating-color sequence.
The Evens Foundation:
Eight piles. Any two can start an empty pile. A card can be added if it's two ranks higher than the pile's current top card and shares its suit — so the only card that can follow an 8 of diamonds is a 10 of diamonds. No pile holds more than six cards.
The Odds Foundation:
Eight piles. Any three can start an empty pile. A card can be added if it's two ranks higher and shares the suit of the pile's current top card — so the only card that can follow a 3 of diamonds is a 5 of diamonds. No pile holds more than five cards.
Cards can't be removed from any foundation once they've been placed there.
Eight tableau piles, four cards each to start, all dealt face-up.
A card can be added to a tableau pile if it's one rank lower and the same suit as the pile's current top card — so the only card that can follow an 8 of diamonds is a 7 of diamonds.
Any tableau card not covered by another is free to move to a foundation or another tableau pile.
Empty tableau spaces are refilled with four cards from the waste. If the waste is empty, four cards come from the stock instead. If the stock is also empty, any card can fill the space.
Only one card moves at a time; sequences can't be moved together.
There's one waste pile, and the rest of the cards make up the stock.
Click the stock to deal one card to the waste. You only get one pass through the stock.
The top card of the waste can be played to the tableau or a foundation.