Victoria 3 needs the pops to be arbitrarily racist andfor cultures to have 'grudges' to have proper historical simulation and to be more fun.
Every campaign consists of liberalizing slowly and the optimal thing to do is encouraging migration into your country. The pops of primary culture will never 'push' against it. They will get only radicalized if they go unemployed, not becouse 'THEY took mur jerb', but becouse 'I have no job'. Filling your 10 million population african country with 1 million of chinese and 2 millions of french should absolutely radicalize the majority primary culture - it should cause some sort of 'cultural friction' - where through cultural shock the cultures that historically have no ties through meeting in 1 state start radicalizing against each other (the strength of the friction should be based on the amount and speed of the arrival of foreign pops).
This cultural friction should be able to grow into a grudge. Let's say (idk just making up stuff, ok?) you are a France and you lose a war against Germany. The French pops in your country should develop a grudge against German pops - they should want to A) implement laws specifically against German pops in France. B) Should get radicalized if you as France (country) improve opinion with Germany or even sign any 'friendly' treaty (never mind an alliance) with them, should grow radical if you don't fight against Germany in diplomatic play for long time. C) Should care less about other minory pops (focus shifts onto the culture with grudge).
Through propaganda the player could increase the levels of friction/grudge or decrease it. The player could spread the propaganda outside his own country to others - different cultures with grudge against the same culture could have less 'friction' between each others, nations with majority of such primary cultures should have better opinion of each other etc. The factory owners of culture X that has grudge against culture Y should refuse to hire pops of culture Y.
This would also encourage playing tall and not painting the map. Since going into war would cause grudges and friction.
Furthermore the majority primary culture should always want to stay the absolute majority, to be privileged over the others and lowering this relative privilege (by lowering the levels of oppression on the minorities) should radicalize them. The game takes place in 19th century, not the 21st.
The game has complete blind spot when it comes to human behavior, it pretends that racism and xenophobia exists in society becouse it's ordained by rules. It sees racism as 'trickle down' thing - where racist institutions and laws are the causes of racism (not the other way around), it believes that people are not inherently racist and xenophobic, but institutions and laws cause them to be. The game of course then shows why such position is naive: if it were the case, no state would ever enact racist laws and as such the states shouldn't have the racist laws enacted on start dates - but historically that isn't the case.