It seems to me that you could create pacts with virtually any type of vast/distant/otherplanar power. The three existing pacts touch on the Feywild, the Nine Hells, and the Far Realms, respectively. A pact with beings of the Shadowfell seems like an obvious extension, not to mention with beings of the Elemental Chaos (and the Abyss) and other domains in the Astral Sea.
If I were to create such pacts for my campaign, I would be very tempted to change the power source based on pact. Why is it that all of these different types of beings are granting 'arcane' power anyway? In the RAW, I just assume that these beings are not sharing their own native power with the warlock, but rather are just teaching the warlock arcane secrets.