Since 5vs5 on paper seems to be very difficult to balance characterwise and OW1 mapwise, and many characters and/or their abilities would be a shadow of its former self (Sombra’s hack either completely gone or it lasts like 2 secs, Ana sleep or nade either gone or their impact/effect lasts 2 or so secs, etc) due to there only being one tank a lot of character abilities would have to be really watered down to the point where it would be bland and basically very plain.
Me personally I would try and keep 6 vs 6 with there being 2 Supports, 2 DPS, 1 Tank, and 1 Flex. The Flex player could play any of the dps and any of the non-shield tanks (so out of the tanks they could only play Dva, Hog, Hammond, and Zarya). When 2 teams go at it you could either see 2-2-2 or 1-3-2. At the start of the match, the Flex player would have a choice to pick any of those characters in those roles and once they select (for example: Soldier) they’re locked into that role (dps role) for x amount of minutes lets just say 2.5 or 3 mins until the timer uplifts (shown on the scoreboard) and they’re able to switch roles if they like. Their team can also see the timer on their scoreboard as well.
This would not affect queue times cause majority of off-tank players are actually dps players as well so it would work out.
If the Flex player picks Hammond for example then the Tank player would have to wait until the Flex player either switches to Dva, Hog, or Zarya or if they go dps so in that regard not much would change as well.
Over the history of OW there have been plenty of different 2-2-2 and 1-3-2 team compositions that have countered each other so in this case adding a Flex Role would be the best solution because it doesn’t cause any limitations towards any team if one team goes 2-2-2 and the other goes 1-3-2 cause at that point what it comes down to is the individual players skill and working as a team to win.