What about keeping everything per the regular rules except for one thing:
Solos get one bonus save (at +0) at the beginning of each turn. The DM can choose which condition they save against. This bonus save can end any condition, even stuff like "until the end of your next turn" or "until end of encounter" effects.
In this way, stuns will be made less severe (relative to other conditions) but with no detailed rules writeup needed. Each turn the DM chooses to save against the most debilitating condition, which may vary. Easy.
It seems like you mean "condition" more broadly than "things on page 277 of the PH”, so I’ll run with that. Letting a solo save against an "until end of encounter" powers risks unnecessarily weakening these powers. For example, it doesn’t seem like the fighter Daily 19 Strike of the Watchful Guard should be something that a solo can shrug off on a successful save, but depending on interpretation it might be able to (what qualifies as a “condition” vs. a bonus to the fighter isn’t necessarily clear).
I ended up using essentially Pale Jackal's suggestion from the previous page of this thread,
in my suggestions on changing Orcus.
Implacable Foe: Whenever an attack or effect imposes a condition (defined on page 277 of the PH), besides marked, [this creature] can make a saving throw (without its usual +5 saving throw bonus) to be unaffected by that condition, even against an effect that doesn’t normally allow a save.
The reason I ended up going with this was simplicity. It’s hard to describe exactly what you mean if you say “status effect” or “anything with an until end of next turn effect” and this is a criticism that applies to the ideas I started with. Defining things in terms of conditions on page 277 of the PH is easy, though not as comprehensive in terms of what solos should be more resistant to (and it allows certain “terminology arbitrage” when a power that’s effectively a stun power doesn’t actually stun the target; e.g., Confusion, Wizard encounter 27).
Consider the contrast between the above and
Font of Solo: At the start of its turn, [this creature] can make a saving throw (without its usual +5 saving throw bonus) against any condition (defined on page 277 of the PH), besides marked, to be unaffected by that condition, even against an effect that doesn’t normally allow a save.
The latter is a weaker ability in general, though the uncertainty about whether a solo will still be affected by a particular condition on its turn can make a party’s tactics more difficult (someone else just hit it with a stun power; should you hold off with the dazing power? Get up near it risking a counterattack if it does recover? It can be hard to decide).
If the solo is hit by a save-ends condition that it really wants to break out of at all costs, this latter ability is a bit better than the above. If there’s an encounter-long condition inflicted on the solo, this latter ability is clearly much stronger, since the above ability only gives it once chance to avoid the effect. I like the timing on the above ability because it’s very transparent; the solo is either affected or not at the beginning and then from there its ability to recover is no better than that of other creatures (besides its save bonus). This minimizes the tactical disruption.
Note that the deity rules don't involve extra saves at the start of the deity's round. Vecna and Tiamat actually have different versions of the ability. Tiamat’s ability is like Implacable Foe, above, but applies to ongoing damage as well and she retains her +5 bonus to save when using it (whether this was an intentional change, or simply bad continuity in design between Draconomicon and Open Grave, I can’t say).
Also, no special rule needed for ongoing damage. Solos should generally have enough hit points not to prioritize getting rid of ongoing damage - but if that is what inconveniences the Solo the most, so be it. Point is: no special rule regarding this save vs beginning of turn needed.
Marks and ongoing damage are two things you don’t want to let a solo more easily shrug off then they currently do. Letting a solo escape marks weakens defenders, and depending on how you implement it this could affect some defenders (because of the way they mark) more than others. Ongoing damage is already less effective against a solo than normal monsters, because the decrease in duration caused by its save bonus typically isn’t balanced out by the increased number of rounds you can expect a solo to survive for (so that it lives long enough to take the damage that’s coming to it).
I should probably add that I consider Orb Wizards to be broken, and that I am not particularly interested to make the rule work especially for them. Much better to nerf those -20's to saves, and nothaving to worry about extreme corner cases like that at all.
I agree. Dealing with Orb Wizards and the AV save penalty items shouldn't be a primary concern here; they deserve a fix on their own merits. I mentioned one for the Orb of Imposition in my first post in the thread; an easy fix for the AV save penalty items (Phrenic Crown, Cunning Weapons, etc.) is that they only apply to the first save made against the effect.