As others have said, I don’t think making polls / votes for deciding which hero to add next would work; more than anything else, because of the time it takes to create them.
Already addressed, but we do have an idea of how long it takes devs to make new heroes, 'cause they’ve mentioned it in AMA’s and whatnot.
IIRC, in the AMA before 2.0 launch they said something along the lines of “it takes 8 to 10 months, Genji was 9 months”.
It’s not impossible that they’ve optimized their pipeline since then, but it’s also true that the dev team has lower resources now, so if anything, I’d speculate development time is longer.
About a year sound right.
And so, the only way I could see something like a poll working is if they had several heroes very close to finished, and they let us vote for which one gets released first (kinda like Hearthstone has done for the Legendary cards in some expansions).
However, I doubt that would be a good idea from the devs’ point of view.
Sure, for the players it’s nice if they ask your opinion, but then there’s also the problem that you know the other options (that didn’t win the vote) are probably gonna be the next few to come out anyway, and that kills the hype for many.
I remember in 2015 and 2016, they revealed heroes in groups (but released them one at a time): first with Lt. Morales, Artanis, and Rexxar; and later with Cho’Gall, Lunara, and Greymane.
Both times, I saw threads saying “there’s nothing new announced for months! ded gaem!!”. One of those threads was literally hours after Greymane went live!
Sure, that kind of post is probably not representative of the community’s general opinion, but I would still prefer it if they released the heroes one at a time, with a teaser a few days prior to get us speculating. 
… Not to mention, at this time they probably aren’t in a situation where they could have several heroes close enough to release-able state to have us vote (we know there’s at least one who we’ll “have to wait a little longer” for).
As for the skins, they’ve done it a couple of times (Azmodunk and Janitor Leoric are the ones I know for sure, and Templar Li Ming is remarkably similar to a fanart I saw before her release).
But I do think they could emphazise more when a released skin comes from the community’s ideas.