Thanks for all the answers, this is very helpful.
I'll try to answer to everything at once:
I don't plan on battling other players for now, this is my team just for the in-game trainers.
IVs
Ninetales: 17 10 31 31 24 30
Aegislash: 31 24 31 31 26 10
Greninja: 30 20 27 29 19 19
Blaziken: 31 31 15 10 30 31
Venusaur: not bred yet, I just have a random bulbasaur at the moment
Garchomp: same
Ninetales: I'll definitely change to Nasty Plot, thanks for the input
Aegislash: I've EV trained it but not leveled it to level 90 yet, so I haven't had a good chance to try him. If I want to replace him it will probably be with Scizor.
Greninja: I outspeed almost everything, and use Water Shuriken for priority. Will changing the nature still allow me to OHKO almost anything ? Aso, I've considered teaching him (toxic?) spikes, but I am not convinced by the utility against in-game trainers
Blaziken: No flare blitz because if I can setup, usually one shot anything anyway and don't want to lose hp. I indeed don't use Brave Bird too often, Knock off / thunder punch / stone edge seem like good replacements. I've chosen Knock off to hit Psychic and Ghost pokemon
Venusaur: I don't really want to use HP fire, because it will take me too long to breed anyway. For the Roserade replacement, I don't agree with the same typing as Venusaur-Mega has fire / ice resistances (Thick Fat). I really like Roserade but I don't feel like they fill the same role... though it would allow me to get another mega evolution (is mega Scizor available ?). Also, is Ingrain really better than Leech Seed ?
Garchomp: I had not looked at the Garchomp learnset and didn't realise you don't get the earthquake TM. This makes me doubt how to build him: defensive with rocky helmet ? Offensive without earthquake ? I haven't started to breed him anyway. Also, Bulldoze vs Stomping Tantrum, I'd go for Bulldoze.
Updated team: