That’s the million dollar question
Until they reveal their game (well, trailer first I presume), we’re left with pure speculation, especially with Riot since their only game so far is LoL.
As for ArenaNet, they’re more into the MMO genre, although their job applications from a year ago were around mobile development. Maybe a game card, similar to “Hearthstone” or “Elder Scrolls Legend”? That would be rather late I think, market is rather saturated now.
Assuming we’re still speculating on a Lumberyard usage from ANet, and that their project is focused on a MMO-like, I hope they’ve learned what limitations to search for and how to best handle some problems.
On “Guild Wars 2”, the PvP format has suffered for 1-2 years of “ghost players” for trains of 50+ players (forgot the technical name for the issue, sorry ).
For the PvE side, some of their world event have been too optimistict on their internal capacity and resulted on huge lags and nobody enjoying the event.
However, apart for those particular points, the game was technically good. On a gameplay point of view, it could spark some arguments, but that’s another story