Is there any (conjectural) characterization of $\overline{\bf{Q}}$-points
on Shimura varieties? 


The question of course does not always make sense
for ${\bf{Q}}$-points: a theorem of Shimura shows that a quaternionic Shimura curve has
no ${\bf{R}}$-points, and a theorem of Mazur shows that the modular curve
$Y\_0(N)$ has no ${\bf{Q}}$-points for $N$ sufficiently large. The question does 
however seem to make sense for certain abelian extensions of CM fields. (For 
instance, in the setting of a quaternionic Shimura curve $M$ defined over a totally 
real field $F$, if a totally imaginary quadratic extension $K/F$ embeds 
into the underlying quaternion algebra, then there is a supply of CM points
on $M$ defined over certain abelian extensions of $K$). In particular, I should
like to know more about the following questions:


(i) Over which number fields $k$ does a given Shimura variety $S(G, X)$
have a $k$-rational point?


(ii) For which such number fields $k$ will $S(G, X)(k)$ be Zariski dense?


(iii) To what extent are such $k$-rational points accounted for by CM points (or similar constructions)?


Sorry if these questions are imprecise or wrongly formulated, I would be happy to at least have an indication of
where to look in the literature if someone has already thought about this.



Characterization of algebraic points on Shimura varieties?
If you haven't, you should first think about these questions just for modular curves, which are the simplest Shimura varieties. Then there are only finitely many $N$ for which the modular curve of level $N$ has genus $< 2$. Once the genus is at least $2$, there are only finitely many points over any fixed number field (by Mordell's conjecture/Faltings theorem). 


For higher dimensional Shimura varieties, once the level gets large enough the variety will become of general type, and Lang's conjecture will (presumably) apply, so as to give restrictions on the rational points. In particular, the answer to (ii) will presumably be *never* if the level is large enough. The $\overline{\mathbb Q}$-points are Zariski dense by the Nullstellensatz,
 and most of them won't be CM points (unless the Shimura variety is associated to a torus, so that every point is CM!), so eventually (i.e. if you make $k$ large enough) you will get points that are not CM. 


