There will be an option on the select screen where we'll have all the alphabetic that we can use to enter a certain number of words/sentences of our own before choosing our character, and when we select the character he/she will be able to speak our words/sentences while playing the match (like Jax in MK:DA).
This could also be saved on the memory card so that we can immediately select the words/sentences that we already have entered instead of writing them again. We can also delete old ones and enter new ones.

This could be done for both playing online or not, but it'll sound better for the online players cus there'll be a variety of people creating their own characters' speeches.

(We press the L2 button for the PS2 to activate the speech while fighting against our opponent).