You can get all legendaries.
But if you have never used an event you can't get Deoxys, Keldeo, Meloetta, Genesect, Arceus, Mewtwo, Celebi, Mew, and Shaymin.
If you don't trade the only ones you can get are: Kyurem, Zekrom, Cobalion, Virizion, Terrakion, Mesprite, Azelf, Uxie, Heatran, Cresselia, Regice, Registeel, Regirock, and Regigigas.
If you don't have Dream Radar you can't get Tornadus-T, Thunderus-T, Landorus-T
If you don't have both Dream World and Dream Radar, you can't get the Legendary with its DW Ability.
Also You can't get Adamant Entei if you Didn't do the Japanese event or if you don't trade.
Hope I helped!