I think a good place to start would be some mechanic that would allow Europeans (or whomever is the most advanced around) to seize and/or purchase port cities without having to resort to total war.
Look at
this map. Look at all the trade cities Portugal acquired in the span of twenty years. Imagine trying to replicate that.
India not being conquered seems like a broader symptom of every war being a total war. In reality, Europe took port cities across India, and when the Mughals declined, Britain and France moved in to expand by pitting princes against each other.
Even by 1821, though, it's a mistake to imagine India as being painted the color of the British tag. There was a lot of indirect rule going on, something that also isn't represented outside of trade companies and vassalization.
There are also edge cases such as the Kingdom of Kotte (in modern-day Sri Lanka) belonging to Portugal because its king passed the title to the Portuguese king. There's no way that this can be simulated in-game.
So tl;dr: this isn't something that can be fixed with a buff/nerf. The reason Europe subjugated India in the first place is complex and beyond what the game is currently capable of simulating unless somebody wrote a really complex event chain.