Sorry, posted before my thoughts were complete. I'm not sure what you mean by diety rules (at first I thought you mean your own solution), but the immediate save rules would not have the +5 bonus. I agree it should not apply to ongoing damage. Similarly, I don't think it should apply to marks.
The deity rules are special rules used for deities, high level solos like Tiamat or Vecna. Among other things, deities get an immediate save to end ongoing damage or any condition placed on them.
Ah. So is there any problem with wording it as, "A solo gets an immediate save [without its +5 bonus] versus any condition, except versus Marks."
That seems pretty good. It's clear what is or isn't covered; on PH page 277, covered; not on page 277, not covered. You might want Attack penalties to be covered as well, but you can't have everything. In return, this doesn't affect "grants combat advantage" type effects or ongoing damage.
You'd need to clarify what happens if a solo gets hit by, say, 5 ongoing damage and weakened (save ends both). Presumably you'd want the save to only end the weakened condition. You'd also want to clarify what happens if a solo suffers multiple conditions at once; I could see going with either "make a save against each" or "make one save against both."
Since it's an immediate save, you'd need to specify how this interacts with the Orb of Imposition- does it happen before a player can use the Orb, or after (of course, as mentioned in my initial post, the Orb deserves a house rule reducing its power and if you use the one I suggest there, this question doesn't arise). Lastly, because this is a actual save, regular save penalties apply to it, which is thematically reasonable. Like the Orb of Imposition, you have to watch out for the AV static save penalty items (Cunning Weapons, Phrenic Crown, Earthroot Staff etc.), which are too powerful, deserve a house rule whether you're using this fix or not, and reduce the efficacy of this fix.