Pure speculation, but I believe the 5 available spots left on the skill bar are tied to future cap increase.

Following the same pattern of skills acquired every 4 leves (and considering there won't be additional cross-class skills spots available), that'd mean there's enough room to get to level 70 before reaching 20 skills + 10 cross-class.

They already stated that switching to a job will reduce which cross-class skills you can use or the number of them, so they will prolly fit among the 10 cross-class skills spots available.