Why do legendary and pseudo-legendary get mega evolutions AND an ability that doesn't hinder them? Meanwhile Slaking has no virtues to playing him when some Pokemon have more stats and more attacks per 2 turns.
First of all, slaking literally has uber-tier stats. Most legendaries have a BST of either 580 (lesser trios like the birds or regis), 600 ("special" legendaries like mew or celebi, and anything in between a lesser trio and a greater trio), 670 (kyogre and groudon), or 680 (most end-game legendaries). Arceus lays sole claim to 720 BST, which is the highest in the game. With the sole exception of regigigas, all 670+ BST legendaries are considered uber. For reference, the highest BST of any other non-legendary is 600, held by the likes of salamence, tyranitar, and hydreigon. The next highest is arcanine at 555 (don't mind the banette there; that's actually mega banette), with starters running between 525 and 535; the low 500s is actually considered on the high end for most pokemon.
So what happens if we give slaking a mega? It gets another 100 BST, and it gets a non-hindering ability. So we now have a base 770 pokemon with a useful ability, which unprecedentedly surpasses arceus. Would it actually be unbalanced? I don't know. Mega salamence is pretty bonkers as is. I doubt it'd actually be stronger in practice than mega mence. However, on principle, I don't see gamefreak giving it a mega, for the sole reason that it would end up surpassing arceus in BST.
Will the newer Pokemon games even introduce more mega evolutions?
They were an interesting idea though a bit imbalanced. Mainly because they gave them to pokemon that didn't need it
Gen VII was decent with alolan forms and Z moves though some things like Island trials were very poorly executed.
