I thought Martinez wasn't the strongest public speaker?
I chose Elizabeth Warren because she embodies everything the Democratic party needs to embody policy wise in 2016. Be for the middle class and not the corporations. The progressive views of Warren will be a huge boost for democrats and would probably sway disenchanted independents. Even if she is getting older I think most Democrats want to be more progressive and not move to the centre right and that her message will fit well. If they choose someone else I at least hope Warren gets a major time slot.
Warren doesn't strike me as someone new and rising for the party considering her age. Usually the speaker is someone who's young and up and coming (Julian Castro, Obama etc.) That's why I mentioned she's young and represents the future of the party.
It partly depends on who the ticket is.
If Hillary has a running mate who represents the future of the party, there's less need to elevate anyone else.
There is another model of a successful keynote: Zell Miller in 2004. Due to his age and the fact he wasn't running for another term as Senator, any future with the Republican party was limited. But a Democratic Governor turned Senator was an effective messenger.