I would like starters to be a little better than they are, specially the ones that are not used often. I would do that by changing their abilities. All starters have the same abilities and they aren't as useful.
This is what I would do to the following starters:
Venusaur
I would give it the Thick Fat ability. It kinda makes sense a bit that Venusaur gets this ability since it is kinda fat but it also helps it to reduce damage from two of its weaknesses.
Charizard
Swap its Attack and Special Attack stats. 109 Atk and 84 SAtk. Since Charizard has a more favorable physical movepool and has Swords Dance/Dragon Dance, it will benefit more from having higher Atk. And maybe, Gluttony.
Blastoise
Take points from its Atk stat and give them to HP and defenses. I would give it the ability to learn Aqua Ring and give it either Swift Swim or Rain Dish. I would also give it Water Spout. It would be interesting so see Blastoise as a healing tank and hitting hard with Water Spout for keeping its HP stat high.
Meganium
Take points from its Attack stat and give them to HP, defenses and maybe Special Attack, similar to Blastoise. I would also give it Thick Fat, like Venusaur, to cover two of its weaknesses.
Typhlosion
Well, a better special movepool I guess. I would give it Energy Ball since it covers Typhlosion's 3 weaknesses: Rock, Water, and Ground. I would also give it Dragon Pulse so that he can cover almost every Pokémon there is. I don't know what ability would be good for it.
Feraligatr
Guts, I guess. This Pokémon is good as it is, its outclassed by Gyarados but thats another story. It could get Intimidate as well since he is creepy looking too.
Blaziken
Technician ability and access to Flame Wheel and Force Palm. Force Palm would have the same power as Close Combat but instead of getting lower defenses and only 5 PP, it would have a chance of paralizing and 10 PP. If it isn't possible to get Force Palm we still got Double Kick. Flame Wheel is just a physical fire attack with 60 power. Nothing special, but with Technician boost and STAB, it could be very deadly. All of these changes can make Blaziken stand out from Infernape.
Torterra
Maybe because its a grass tank, I choose Thick Fat for this Pokémon as well. It's x4 weak to Ice so it might need it.
That is all I can think of. For Swampert, Sceptile, Empoleon and Infernape I would choose different abilities but I don't know which would be best for them.