I understand that close reasons are supposed to be as clear as possible to the OP, whilst simultaneously not overburdening the person suggesting the close with too much navigation through multiple close reason pages.
So at present to suggest moving to another site I click:
Close > Off-topic > Belongs on another site > pick site > Sharepoint!
BUT (!) frequently the list does not contain the site most suited to migrate to (how many questions get migrated to meta nowadays out of interest?). Is it really too much to ask that people asking/answering questions on this site click a button marked other at the bottom of the site list to be taken to a full list of sites? If this is too much, can the migrate option be moved to the top level of close reasons so that this extra step does not obfuscate the close issue? So the process would become:
Close > Belongs on another site > Pick site > Other site > The right site
I personally find it incredibly frustrating that sites such as Cross Validated are not on the list, despite having ~23k questions whilst both Database Administrators which has been running for 6 months longer than Cross Validated, has only ~ 15k questions and Sharepoint (?!) also has around 26k questions (but has also been active for 6 months longer). I believe the off-topic list could be more flexible and that would give the best directions to OPs about where their question belongs.
I think that as there is a certain barrier to entry to get anything out of the site (the ability to ask a well-formed question or the knowledge to answer it) that this increased flexibility would in no way make the site worse. I do admit I come from the data domain side of things (so Cross Validated would be very useful to have) and users from other disciplines may vehemently disagree with me, but I hope I have made a clear case for offering increased flexibility in the site migration options.