Wouldn't be a proper set without being certain about a few initial considerations:
1. Size of the set? Large, small, or an unconventional size?
2. Themes? Story & playstyle both. Seems that with the cards you've already made, multicolour is a very prominent theme, as they're usually very few in number in usual sets. With exiling being prominent, there would also have to be a graveyard-focused subset as well so that exiling is meaningful. Maybe some library manipulation as well, but not on the level of dredge.
3. Number crunching of the set. Cards are organised in sets by the set number, which is sorted by colour and rarity. If rarity isn't a consideration here, then there will still need to be a proper colour balance. I suppose you could also model it after Torment, where the number of blue cards in this set is reduced, while red and green cards are more plentiful. Black would focus on graveyard and white on exile, I think.
4. The format. Will this set be balanced for block constructed only? Limited? Modern? Balancing for cube might be the best option, as conventions can be disregarded for the purpose of building the best set of cards with which to construct a cube.
5. All of the above serve as guidelines for a unified vision that plays properly and "feels" like a set. Without any direction, a single person working on a single idea for a set might not even be able to design subsets of cards that converge upon that idea properly.