current ideas for solitaires:
quaternion themed stacking rule thing where there are negative ranks and the foundations intersect at zero
some set packing thing with factories like triqueta or azul
intersecting foundations might be interesting in general. brick solitaire
i think i have something with the latter

the whole deck is shuffled into a closed talon pile, which is turned over X-at-a-time into a waste of X cells, under which are Y depots. the talon can only be turned over when each cell in the waste has one or no cards; cards stacked upon another card in the waste can be moved to empty cells in the waste.

cards may be moved one at a time and placed upon cards in the depots that differ in rank by one (ascending or descending); packets of two cards of the same suit may be picked up as one and placed with the rank of the bottom card.

whenever there is a packet of three or four cards of the same suit, the packet immediately goes to the foundation, stacking on top or underneath corresponding packets already there. there is no grace move to move individual cards to the foundation when these packets are not adjacent, nor is there one to correct the direction of a packet. every suit must be packed in the foundation in three packets of three and one packet of four.

update: instead of separate wastes and depots, have N piles that can be dealt to from the talon if they have less than two cards on them, and also make it so the foundations are strictly in descending order. i'm tempted to have 3 suits of 17 ranks for more move blocking and euclidean division nonsense respectfully (17 = 3*3 + 2*4) but i don't know what i'd do with 17 cards to a suit. 9 numbered cards with 4 major and 4 minor picture cards?