Wow, that’s actually some great news for Mercs, which is currently one of the few things in the game worth playing!
First of all, about the plans for new content. Actually, I’ve already written about it before, how the ‘quantity over quality’ approach just doesn’t cut it. ‘Once these Mercs join the fray, we’ll have over 120 total Mercenaries for players to collect, add to their Parties, level-up, and max out. That’s one bustling Village!’ — exactly. Besides, as I’ve also said already, I’d very much prefer at this point that they’d work on fixing the many bugs and issues present, rather than drown it all in waves on new mercs and zones, as they have done before. Thus, I actually think it’s a right direction for them to have taken. Of course, there’s a long-term question of what happens if, or when, the Mythic mode gets ‘old’ — but we’ll see about it. If it’s just endless new puzzles to solve, then why would it? That’s exactly what Mercs fans are looking for — but, once again, time will tell how it’ll work in practice.
As for the factions, dual-type mercs and all this — I like it! That’s exactly what’s been somewhat missing and worked in a strange manner: the Alliance and Horde, some pirates missing their base races and so on.
Interesting thing about the ‘Mythic Boss Rush’: I’ve been thinking how I would implement the PvE end-game system, and a gauntlet of bosses with some buffs, extras etc was more or less the first idea that immediately came to mind. So, it seems they’ve taken the most logical direction in this regard. Oh, same thing with excess coins, or ‘renown’, by the way.
A couple of other remarks and ‘disclaimers’. First, I’m writing this review based on what has been announced — the actual implementation has not always delivered everything that’s been promised, especially bug fixes, so it remains to be seen how it works. Second, I’ve avoided discussions about potential leaks from data-mining and such, since it’s apparently against the forum rules.
Oh, one last thing: I know there’s a marginal group of dedicated Mercs haters or clueless bypassers with an irresistible urge to inform everyone of their ignorance on this forum, but to every one of those I’ve got this to say: ‘You’d make a cute puppet.’ 
Oh, and to the hyperactive types who invested a lot, then got FRUSTRATED and joined that bunch: thanks for you money , your opinion… is very important to us.
(A few minor edits made.)