People aren't generally keen on failures, and doing a "training run" means people will think she's a failure, an also-ran. Assuming she gets the nomination (otherwise people won't care).
It's pretty standard for potential future candidates to make such bids. Especially if they have significant ideological differences to the front runners. I mean, that was Ron Paul's whole shtick in '08 and '12. Same for Kucinich on the Democratic side in '08. You usually get a bunch of candidates who have no chance of even making a showing, but who run the early primaries/caucuses. Given Warren is already up there with Biden in early polling I'd say she has a chance of hanging on for a while longer than most.
More significant for me are the comparisons to
John Edwards, who literally dragged the field left ahead of the 2008 primaries (including shifting Clinton by 18 points on the On The Issues measure). Edwards is gone, but Warren has the
populist cred to fill a similar role.
Also worth noting her age. She is only two years younger than Clinton and there are suggestions that Clinton's age is already a factor (admittedly combined with her recent health issues). She might even see this campaign as her only chance to make a play on the national level.
Is there a similar list somewhere of possible republican candidates, or are they too numerous to really count?
This is pretty comprehensive, but we are talking about a party that made Sarah Palin their VP candidate in '08, so I wouldn't put it past them to have a couple surprises. My understanding is that Chris Christie, Rand Paul and Jeb Bush have all performed well in early polling, but well here means
barely double digits, with a host of lower name recognition candidates sitting at 5-9% behind them.