Yeah this is a terrible idea. If you sided with Trump, there is no forgiveness. No second chances.
Romney took a brave stand in the 2016 election. He repeatedly and loudly denounced Trump. He did the right thing.
Then Trump won, and Romney allowed Trump to stick his tiny mushroom-shaped orange cock into him.
A "national unity ticket" is a noble idea. But not Mitt Romney. Not anyone who ever made peace with Trump.
Not that Trump will make it to 2020, but if Democrats want to win, the top of the ticket should be an experienced centrist Democratic figure.. probably in their 60 or 70s, White and Male and able to speak to the working class and with "executive experience". His running mate should be a younger figure... probably in their 40s. Probably a minority, biracial and/or female. This person would be anointed by Saint Bernie as the progressive standard bearer, and would speak to younger people and the growing diversity of the Democratic Party.
Ideally for the party's sale, the candidate at the top of the ticket would only serve one term, and make room for the Vice President to run for President in 2024.
Here's the Democrats problem... and it's the same problem that Republicans faced back in 2011 (just before the 2012 election). In 2008, both Hillary and Obama had an enormous bench of talent to draw on, because the Clinton Machine and DNC had spent around the years 1996 to 2008 building it. Similarly, the Republican Party in 1999 had a huge bench of talent to draw from that it had been cultivating since the 1980s.
But you get to 2011? Bush looted and pillaged the Republican bench, and his administration tainted so many careers, it let the tea party and the other crazies walk right in. Recall how many losers and charlatans were on the stage in 2012 at the Republican Primary. It only served to make Romney unconvincingly say crazy things.
The Democratic Party in 2016 and 2019 is in much the same spot. The Obama years looted and pillaged the Democratic bench to staff the administration, and did very little to build the bench like the Clinton Administration did. In fact, the DNC had a rather hostile relationship with Obama's faction for just this reason. The most important political job for President Hillary Clinton for her party was going to be standing up a new generation of Democrats ready for prominent positions, including high office one day.
Now the next Democrat will have to do that, and the pickings are thin. Between the Octegenarians who rule the House, the Governors without National Appeal, and Obama's team scooping up the next-gen talent without replentishing it... there is like a big hole of ready-to-lead Democrats in their 40s and early 50s who aren't Senators.
This entire thing I've described... it's nothing new. It's happened before. But it takes a good 15 years to reboot the parties. Democrats will have to do it in about a total of six years. Not easy.