Assuming a plugin architecture where the plugins form a single program with the kernel, all parts of the system (kernel and any plugins) must be available under a GPL-compatible license. The MIT license is GPL-compatible, and is therefore allowed.
For a list of possible licenses, see the GNU list of Various Licenses and Comments about Them. Note that GPLv2 and GPLv3 are incompatible, and other licenses might be compatible with one but not both of these GPL versions. If there are plugins that use GPLv2 code and other plugins that use GPLv3 code, this might be a good reason to choose a license for the kernel that is compatible with both (e.g. GPLv2+ or MIT, but not Apache 2).
The GPL FAQ has a number of questions regarding plugin architectures, starting with When is a program and its plug-ins considered a single combined program? The FAQ explicitly discusses the case of plugins to a GPL program, but the reverse (GPL plugins to a non-GPL program) is equivalent.