* time wise - are you okay adding the extra 5-150 seconds a player uses to decide between at-wills? (if your players take time in such decisions it would be towards the longer end, every round, times as many players as you have that make decisions this way)
* How long do your combats typically last (in rounds, not talking real time) ? At higher levels, you have several more encounter and daily (and magic item daily) powers, so at-wills see a lot less use. Example: an encounter that lasts 5 rounds is going to get maybe a couple encounter powers, one daily power, and one atwill power. It may not be worth the effort to do the extra atwill if it won't see that much use
* do your PCs most often revert to one of their two at-wills as a default, with the other being for special cases? If so, you aren't really adding anything other than an encounter power disguised as an at-will.
* Mechanically, some classes (like controllers since their control features are more situational) and something like the barbarian (that does a lot of charging and has a couple at-will choices that can be used when charging) will get more of a versatility boost than a class like the rogue.
having said all that -- while i haven't given this much thought beyond the 2 minutes it took me to type this response, i don't think you'll really "break" anything. I think it'll just be a matter of preference with regards to "are you willing to take the extra decision time" etc. As long as everyone knows the change before making their PCs and is okay with it, you should be fine (though adding the change after the fact may annoy someone who unknowingly picked a class/build that would get zero benefit from it).
As far as the ritual caster feat increase. i have to admit, i too am biased in liking the fact that it is opened up. but maybe instead of restricting the feat to two feat slots (which can make for messy book keeping), you restrict certain rituals in groups, making it a ritual caster feat per group...
something like one of these three alternate versions (i'm saying to use one of them, not all of them at the same time).
Ritual Caster (alt version 1)
Prerequs: whatever the prereqs are in PHB (I don't have it handy
Benefit: Select one skill that you are trained in (arcana, religion, nature, etc). You can cast rituals that have that as the key skill.
Special: you can take this feat multiple times, but must select a different key skill each time
-- keep in mind that all skill associations are not equal (some skills like arcana have far more rituals than religion)
Ritual Caster (alt version 2)
Prerequs: whatever the prereqs are in PHB (I don't have it handy
Benefit: Select one category of ritual (exploration, scrying, creation, etc). You can cast rituals in that category.
Special: you can take this feat multiple times, but must select a different category each time
-- keep in mind that all categories are not equal (some groups have far more than others)
Ritual Caster (alt version 3)
Prerequs: whatever the prereqs are in PHB (I don't have it handy
Benefit: You can cast rituals of level 10 or lower
Special: you can take this feat multiple times, each time you take it, your level limit of rituals you can learn is increased by 10 (i.e. you have to take this twice to be able to learn level 20 and lower rituals).
but, anyway, all that is getting in to house rule territory so I'll stop there.
