pokemon base stats are now the stats of its base form *2, *3 if its a 3 stage evo
example
venusaur's new stats:
135
147
147
195
195
135
how does the meta change
>>31667381
charizard new stats
117
156
129
180
150
195
and blastoise's new stats
132
144
195
150
192
129
3 stage evos get insane boosts
2 stages get fucked
>>31667381
There would literally be no purpose to single-stage lines anymore, as they'd have a shit BST compared to everything else, or they'd have to be retconned into legendary levels to keep up the pace of mons that do evolve and get their stats doubled or tripled. This includes stuff like absol and heatmor, and pretty much every single actual legendary in the game.
Two-stage lines would be extremes. Mons with shitty stats that only evolve once produce final stages that are horrendous (Sunflora's stats: 60-60-60-60-60-60), while mons with decent stats that only evolve once produce absolutely terrifying monsters (Scizor's stats: 140-220-160-110-160-210).
Finally, 3-stage lines would produce similar situations. Gengar would have 90-105-90-300-105-240 in stats, for example. And let's not get into mega evolutions.
Everything that evolves 3 times becomes a fast wall, wallbreaker and sweeper at the same time, except single stages and specific mons that are pushed forever into non-usable territory. If moves and abilities are not changed at all, many pokemon can do pretty much every role and the meta becomes more a matter of what typing/move/ability combos you can make, with the actual stats not mattering that much since everything is so high.
TL;DR - it's a stupid idea and you should feel bad about it OP
>>31667381
As an expansion of the point made by >>31668109, let me take my two favorite lines, Torchic and Ralts.
Torchic ends up becoming a 135/180/120/210/150/135 as Blaziken, which is only not ridiculous because so many other Pokemon are even worse monsters, like the aforementioned Gengar. If we're using the same logic for Megas, a Mega would be considered an extra stage, which would bump the stats to 180/240/160/280/200/180.
Ralts presents problems of its own; assuming it evolves to Gardevoir, the stats before a Mega are 84/75/75/135/105/120. Much less broken; outside of a massive Speed jump it's not appreciably stronger than it was. The Mega would bump it to 112/100/100/180/140/160, which is considerably more nuts, but the biggest issue here is you've left Gallade with literally no reason to exist.
Dragonite: 123/192/135/150/150/150
Tyranitar: 150/192/150/135/150/123
Salamence: 135/225/180/120/90/150
Metagross: 120/165/240/105/180/90
Garchomp: 144/210/135/120/135/126
Hydreigon: 156/195/150/135/150/114
Goodra: 135/150/105/165/225/120
Kommo-o: 135/165/195/135/135/135
Blissey: 300/15/15/45/195/90
Aegislash: 135/240/300/105/111/84
Lunala/Solgaleo: 129/87/93/87/93/111
>>31667381
that's fucking stupid