@Mary it is definitely on my to do list.¶
Marriage is on my todo list if you wanted to say that you wanna get married.

Do you want to marry or do you want to tad someone named Mary and this is a response to her? It became too confusing because of @ that you added in the beginning.

I will come to visit one day. ¶

what you wrote is correct but people leave the 'to'
and it's taken for granted.

I know that there is only a verb in an English sentence, but in the example sentence, come and visit both are verbs, and this sentence is from an English native speaker.

"visit" can follow "come." Remember that "come" and "go" can, conversationally, be followed by the simple form of another verb.