Not
quite correct. The problem I'm referring to is that, once Kuro split off, Ilya's prana capacity apparently took a nosedive. Whether it's because Kuro unconsciously taps most of Ilya's reserves to sustain herself, or whatever method formed her just
took most of Ilya's Magic Circuits and placed them in
her (pseudo-)body is never explained - nor is it ever explained
how the problem fixes itself. However, (at least in the
anime) after a couple of fights where Ilya collapses due to magical exhaustion, the problem just seems to
go away. At least, it's never brought up again.
. . . As you might have guessed, I take issue with this. As a result, Prisma Ilya's capacity is going to
stay diminished (unless the Works finds a way to repair it), and she's going to
require Kuro/Ilyasviel's presence in order to survive her coming battles. This, of course, will necessitate her leaving - which is its own set of obstacles to overcome - I'm mainly thinking of the need to transfer the
geas to someone else in order to allow her to return; preferably someone compatible with, or connected to, Kuro in some way.
But still, that leaves the question of, who should she be replaced with?
Should she be replaced? Shirou's presence fulfills my "one Type-Moon/Works character, one OC" partnering mandate, after all; on the other hand, Chiwa Saito has voiced an
awful lot of Type-Moon characters besides Kuro, including Tamamo-no-Mae . . .