Seems like you just want to keep playing d2, but with flying mounts.
Rift was great when it came, same was Archeage - both games are ruled by Trion which fucked over both games.
WoW is too old, don't go there. You can't fly in the new expansion.
Guild Wars is actually fun, and so is ESO, but you don't have open PvP which is sort of a dealbreaker. (Although both of their pvp places are really fun and makes up for the lack of open-world pvp, especially ESO)
TERA got true action combat (no tabtarget bullshit like most other games) and you can kill people openly (Everyone.) Actually quite nice of a game, but you want to play on NA servers so if you live in EU you'll get delay and that's boring.
Wildstar is like a ramped-up disney hackNslash game also without tabtarget, was really fun, not sure how it is now.
Didn't really know Neverwinter was still a thing, same with Lineage - They seem to be damn old, but not hanging in there in the same way that WoW does.
You have lots of gear in every game, but just like every game there's only so many pieces that actually matters. Same for D2, you won't have a metric crapton of gear that is equally good, they always come in different tiers.
This also goes for the PvP thing you talk about (there's always going to be a meta-game), but if you want to be versatile, ESO probably is the best one for you, with Rift/Archeage as runner-ups since you can have one characted develop into different roles (ESO and Archeage they can take any role you want, for a reroll fee), the rest of the games are mainly "Chose one of 1/2/3 specs"
Look at different games on youtube, "PvP gameplay" or whatever and see for yourself what it looks/feels like. Gonna be a lot easier than writing 10 pages on jsp about the games.