I suppose the Compact could be exactly the thing described in the Tribunal Temple's articles of faith - "By the Apotheosis, the Tribunal (Blessed Be Their Holy Names) became the Protectors and High Ancestor Spirits of the Dunmer, and bade the Daedra to give proper veneration and obedience. The Three Good Daedra, Boethiah, Azura, and Mephala, recognized the Divinity of the Triune Ancestors (Blessed Be Their Holy Names). The Rebel Daedra, Molag Bal, Malacath, Sheogorath, and Mehrunes Dagon, refused to swear fealty to the Tribunal (Blessed Be Their Holy Names), and their worshippers were cast out". As you see it, 7 of the 10 Compact parties are actually the Dunmeri Anticipations and the House of Troubles with only Mora, Hircine and Vile left unmentioned. That could be somehow connected to the Tribunes and the answer might be found there. Maybe it is all about his Clockwork City he began to build on Sun's Dusk 31st 1E 2920 - "In the smoky catacombs beneath the city where Sotha Sil forged the future with his arcane clockwork apparatus, something unforeseen happened".

Regarding the clauses. All we are left to do today is to theorize, indeed. I prefer to build my own opinion based on the knowledge of some facts already portrayed in the games - I think that the price offered the Lords for their personal absense on Nirn could be the permition to have the very influence, personal spheres they have on Nirn today. Well, they could be offered something else too, and what I personally think of the Compact is that it is not hard to imagine what else could be offered to each of the Daedra Lords - it is much harder to imagine how could Sotha Sil manage to perform his part of the deal (if he ever intended to) without theorizing. These are just examples I stopped at:
1. Hermaeus Mora knows everything except.. the Elder Scrolls. I mean, the items. I might be wrong on that, but as far as I remember the Scrolls disappeared from the Imperial Library in 4E 175 and were scattered all over Tamriel. If that is true, that could be the first step towards the next TES game or an ESO DLC portaying his pursuit to get them;
2. Sheogorath could demand Almalexia - he likes to make persons mad instead of masses, but never before could he claim a soul of such a scale as hers in his possession through making such a creature as a living god insane;
3. Hircine could be granted the right to host his Great Hunt and manifest himself in the Mundus once every era - something shown in the Bloodmoon. A fully legit extra hunting ground and I think he did it without breaking the rules, because it could be the very thing he was offered along with lycanthropy ever existing on Nirn;
4. Azura could be offered the Heart of Lorkhan because it seems to be the only thing she has been really bothering about since she buried it in the crater of Red Mountain (read about it here if you have any questions on why it was she in one of the lore lines or find it interesting);
5. Mehrunes Dagon could be promissed Tamriel and power of it's natural disasters.
6. Molag Bal could have asked for souls and an ability to create the undead, vampires and all such creatures on Nirn;
7. Clavicus Vile, if he entered the Compact instead of some other agreement, could be granted the right to harvest souls during the Red Year - something he did when Baar Dau began to fall;
8. Mephala "perceives all of Aurbis as an interconnected system of action and consequence, and employs herself in spinning new threads to influence outcomes" - something very similar to Sotha Sil's perception he told us about, though except that "to influence outcomes" part. He could share.. his knowledge?.. Anyway, I have no good answer to that.
9. Boethiah and Malacath - no idea. Possibly later DLCs and games will show us that. Since the two were connected to each other in their past, I suppose they could ask something directly controversal. But it's just a guess just like the previous 8 points.