I might be wrong but as the options for FINAL professions I would say:
Alch - 300 Classic, maybe TBC?, 75 Northrend
Engineering - 300 Classic, 75 Northrend, 75 Cata, 25 or 40 Pandaria(?), 1 WoD
Jewelcrafting - 300 Classic(?), 75 TBC(?), 75 Northrend
Anyways it really depends on whether you're f2p, vet, or still subbed.
If you're either subbed or want to use the mark of honor gems, you won't need alch and jc to make better gems.
If you're subbed then you don't need 75 cata engineering or 40 MoP engineering for socketing dragonling
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so if you're subbed I would probably say your final professions would be looking like:
Engineering
Inscription/Alchemy
No specific profession level for subbed because as subbed you can just mail yourself stuff or whatnot and don't need to be TOO cautious in gathering mats so you don't have to reset a profession to get more, and you have easy access to mats
don't quote me on anything above because I don't play vet and am relatively new to F2P, just my thoughts