Communism would be the biggest one that leaps out at me.
As an ideology, communism could be pointed out as assimilating ideology. But you could also say that many ideologies are similar. If a communist become a capitalist will he be viewed with suspicion? He will be welcomed as a new "convert". All political ideologies prefer to have as many new "recruits" as possible.
Proselytizing religions also behave similarly. Islam is an example. If one is converted into Islam, he is usually not discriminated on the basis of his original background. But, Christianity, as it has undergone several changes in its development over the years, do not exhibit a similar degree of elasticity in accommodating the new converts.
It is not in the political ideologies or religions that we must look for assimilating capacity. It is the national cultures of the countries that must be studied. Whether the states or countries have the capacity to accept and assimilate the new groups of people that enter their borders.