I-6 is a possibility for that Freer-Corpus Christi route in Texas, even more so if the entire SH 44 route from US 83 (which could be future I-27) to CC is upgraded in the future.
I-34 or 36 could be routed on a future upgrade of US 74. Never doubt NC's ability to add new Interstates. 
I-9 is a possibility in California.
Just curious where 9 could fit in California. I thought if CA 99 was an interstate it’d be 7.
Could be 7 or 9. Either number fits.
The CA 99 corridor would probably be I-7. I saw someone post here one time, "They need to renumber the western I-84 as I-82 and I-82 as either I-7 or I-9, since it mostly runs north-south."
82 drives me nuts. It’s a north-south route!
It's also north of I-84.
To be fair, though, the highway that is now I-84 was originally signed as I-80N, so I-82 fit in the grid back then. If I were to complain about the number change, it would be that I-82 should've been extended south and east through ID and UT.
And I would keep the 84 numbering since x4's are next in line behind x0's (I-44,64,84w,94) as long distance east-west interstates. For odds, 9's would follow the x5s (I-29,49,69) though a similar argument could be made for the x7s (I-57,77).