An arena for group duels would be great or at least the option to do it in housing instead of having to do it in cyro or ic dealing with trolls.
As far as bg’s go though if anything I’d say less modes or at least the ability for a dm que since being forced to play modes people don’t want to play has reduced the population.
Unfortunately the lack of attention to the performance issues has severely reduced the pvp population so adding too many options would just split the community up even further. If they did fix performance I think the community would start growing again, but until that happens the population is going to keep decreasing no matter what they add.
I think they recently made available again the house that permits PvP. I cannot recall the name but there was a series of weeks they offered previous homes.
Someone suggested in a different thread that ESO may not have the PvP population to make doing significant work worthwhile. That comment may apply here.