formally Death on Wings
Concept: A Pokemon that poses a great defensive threat before Mega Evolving that Mega Evolves into a large offensive threat. Another plus of this concept is that it might be better to hold of Mega Evolving at times, to keep the defense. Venusaur should keep in its normal form during the largest part of the game, only Mega Evolving when it can sweep the opponent, or at least heavily damage them.
Competitive Justification: It'd be cool to have a Pokemon that can be both defensive and offensive within a single match. It's also cool to have something that has a good reason to not Mega-Evolve for a great deal of the match. I think it's good for the Pokemon as well as it can do stuff like support the team or setup before it Mega Evolves, and it'd be interesting to see how people would stop it.
Flavor Justification: Honestly I turned this one around. Normally it poses a better offensive threat in its normal form (Chlorophyll), though not a great one. Mega Venusaur is a great wall. This does show that Venusaur can be both offensive and defensive, which is shown by its normally balanced Stat Spread as well. Capitalizing on this would be a good concept to build around as you can go a lot of ways.
Charizard Winning ConceptPokemon: Blastoise
Concept: Bulky Rapid Spinner
Competitive Justification: Blastoise as a bulky spinner why should it be a bulky spinner? I feel it fits it perfectly because looking at its design its a turtle and has a giant shell its reflected up by its defenses with 79/100/105 bulk. It is let down a little bit by its small Hp. I feel it is let down by only having one typing, the other two Kanto starter both have dual typing that fit them. Charizard has wings it got flying type as its secondary typing. Venusaur is a poisonous plant it was given poison typing as its secondary typing. Now what would help Blastoise need Steel typing. Why Steel typing you may ask why? Look at Blastoise back he has steel cannons on its back its very fitting for it. Steel typing would also benefit it as a bulky spinner. Steel typing would give it many new resistance's such as Bug, Dragon, Fairy, Flying, Normal, Psychic, Rock, Water. Then giving it a dual resistance to Ice and Steel, and an Immunity to poison. It also gives it new weaknesses electric, fighting, and ground. Honestly I feel blastoise needs to become a better bulky rapid spinner.
Flavor Justification: As a turtle it should be bulky, also taking its shell into account should be bulky. Then looking at its steel cannons it should be part steel.
Concept: A Pokemon that forces mindgames with the opponent which Pokemon it'll mega-evolve into. The mega-evolutions will be very different and a misplay by the opponent can have major results, due to the different strengths of the Mega-Evolutions. This will cause your opponent to have to be able to read you well.
Competitive Justification: This is cool because it forces you to think well which Pokemon to use to counter it. It might be pretty easy to beat with the right Pokemon, but able to easily beat Pokemon that are lured in by the other Mega Evolution.
Flavor Justification: It has two relatively different Mega Evolutions, even if they have quite similar counters (Heatran, Azumarill). It's also one of the few Pokemon with multiple Mega Evolutions, so one of the few that can execute this concept, and the only one that's not legendary.
While this might be the optimal typing for a bulky spinner, I'm not sure how well it suits the flavour. Honestly, I think beating being toxic'd and beating toxic spikes could be done perhaps by having heal bell, allowing it to support the team further, while usable (though not overpowered) offenses would still allow it to do decently against spinblockers which are. I don't know how much flavour should influence what we do with these pokes, or how much we could influence the flavour (I could imagine changing the flavour of Mega Blastoise to give it this new typing), but I think being able to do these things without such a drastic change should still be possible.I'd like Blastoise (Or Mega Blastoise) to be Dark / Poison. It allows it to excel at the spinning part due to the Dark typing, it can hit spinblockers super effectively, but most importantly it won't have to have good offenses as it gets access to STAB Foul Play, which is excellent for a defensive Pokemon. Poison would be cool as it gives an immunity to Toxic Spikes, which is important for the spinner on a defensive team, as it's bad if you get inflicted with Poison while trying to spin them away as a bulky Pokemon. It also gives excellent type synergy with the Dark types, resulting in only one Weakness, which has a lot of immunities (we could even give it one, later!). It also checks Psychic types really well.