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Well, what we haven't factored is the measure of powers at that level.
For instance a power that grants +4 to hit and deals 6[W] damage at Level 25 might grant +32 to hit and 48[W] damage at level 99.
Likewise a power that lowers an opponents AC by -2 at Level 25 might lower it by -16 at Level 99.
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This is indeed an important factor; and one that will be the most difficult to approach.
On the one hand, you could have powers scale in damage, but if you make them scale in other things, (like the mentioned AC penalty) the game could break down like 3E breaks down to straight Sucess/fail.
Example: Lvl 99 with that -16 to AC power (hypothetically) - If you land this power, that opponent will die. The fight will become grossly easier with powers that destroy the 4E power scaling (5X so against a solo enemy), Unless the DM just ups the foe's AC by a similar ammount, which means landing said power will now be impossible, and the fight will be unwinnable.
The factors of powers that I think should scale:
* Damage - Needs to, and linearly, since no exponential HP explosions. Also, base weapon damage should not scale, but powers that use weapons should deal more X[W].
* Movement - Once deities are involved, scales need to increase. Especially at the cosmic level. It would be awesome to see a (cosmic) ranger shift behind a planet and use it as cover.
* Area - Needs to scale if movement scales. (Cosmic) wizards need to be able to hurl sun-sized blasts to catch those pesky cosmic rangers.
* Range - Same reason as move/area.
* Ongoing damage - easy to scale, ~5 damage per 10 levels.
* Resistance Piercing - Should not scale terribly fast, but should allow high level characters to of-handily defeat far lesser opposition.
* Depletion of Healing surges - This is really for monsters, but some monster (The Soldier-Wight, as an example) can just deplete healing surges directly on a hit. This should scale but very, very slowly because Healing Surges are linked to Con Mod, which could be a scaling attribute. A Charater with 50 Healing surges is not going to be inconvienced by losing 1 healing surge, and probably the other characters will have around 20 by then, so draining 2 at once might be in order by then.
Factors of powers that do not need to scale and I think interfere with a linear power progression:
* Powers granting increasing penalties/bonuses - Breaks a linear scale. A bonus to hit of +2 - +5 is enough to turn an average hit rate into a very high hit rate. A bonus to hit of +10 - +20 means you cannot miss except on a 1, or that the foe you are fighting could not have been hit without the powers. (or a clunky middle ground in-between) Some powers already scale like this (Armor-splinter - Ranger 13?; penalty equal to Wisdom Mod to foes AC) and need dealing with.
* Status conditions - Adding new conditions is fine, but adding more conditions to each power with each new tier of power is not needed. Conditions are far stronger then damage. I ran a ranger the other day at a friends game that was completely neutered by Immobilize, Daze, and ongoing damage hitting him every other round. A single attack that deals more than 2 conditions is probably too much.
* More powers: ~3 At wills, 4-7 Encounter, 4-7 Daily, and about 10 utiliy (of mixed use rates) are probably enough powers for every one. At level 100, we don't need 10/18/18/37 powers for each character. 18 encounter powers? You could never use them all! (If you can, your fights probably last about 8 hours each!)
* Number of attack rolls - After a certain point, this leads to absurdity. 2-6 attacks is more than enough for one power.
Now, the chief problems I can see with infinite (or extreme) extrapolation of the current rules are powers that violate the views I note above. Example: Armor Spliter - A penalty greater than 11 means you can't really miss. Not quite possible before level 30, but other powers can combine to do it. If you keep scaling penalties/bonuses, then you need to give all monsters more defense, which means all the powers need to grant bonuses and penalties to even initially hit, which means you probably should just cut out the middle man and skip scaling penalties.
I am sure there are many powers pre-30 that, if allowed in a level 31+ environment would break the game.
A simple solution is to provide some sort of tier-immunity, and to make sure the powers you create for immortal's levels don't break down.
Example:
Say the tiers (of play, not of levels) go like this - Mortal - Divine - Sidereal - Eternal
You could just make a simple Sidereal trait mortal-immunity - Mortal-level powers are useless against them (auto miss, grant no benifits in their presence, etc), and Eternals might have Divine Immunity.
By the time the PCs hit the Sidereal tier, their abilities from the mortal levels should start to be breaking the game down. Begining to encounter Sidereals with their immunity is a a mechanical incentive for them to trade up their 'Armor Splinter' (-~20 AC) for the Divine power Deatomizer (Far more damaging than Armor spliter, -4 AC to the foe, but still -2 on a miss) which, while it doesn't grant nearly the penalty, will deal appropriate damage for it's level.
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