These are all the Pokemon you can't get with the games you have(this includes forms and regional variants):
**All Unova, Kalos, Hoenn, and Alolan starters and mascot legendaries that you didn't pick, or didn't have the mascot for.(This obviously requires trading or reseting)
White Kyurem(unless you trade your Reshiram from US to B2, and use your Kyurem there, using GTS)
Latias(If you don't have the event item)
Eletrivire, if you decide not to trade, the item's available, but you would have to trade
Dawn Wings Necrozma
Note, if you pick Archen in B2 and Aurorus in Y, this list will apply, otherwise, them and their evolutions should be considered added. This list also doesn't include all Pokemon that can be obtained through breeding, Poke pelago, or Pokemon that can only be obtained in the Gens/Regions you don't have, I, II, VIII. If you want I could include these.
Source: Cross referencing every Version exclusive Pokemon page with the locations page of those Pokemon, as well as Serebii, as it includes some things this site doesn't(Fossil Man in Twist Mountain, since he gives you the fossils, not the Pokemon, and 0RAS Legendaries through the Mirage(Lake Spirts, etc) also, Knowlege(I've played Sun and OR before)