Monolith Soft is probably one of the best development teams out there and, as for every good thing, development of games at the level of Xenoblade Chronicles and Xenoblade Chronicles X takes a lot of time.

Considering the facts that:

-Monolith has recently gone through massive recruitment for developers;

-Monolith usually gets involved in various other projects by Nintendo, in fact many speculate that right now they might be helping with Zelda U;

-devs at Monolith as they've shown with both the Xenoblade games likes to challenge themselves with new and bigger worlds so their next title will most likely be enormous;

I personally don't see them releasing their next big title earlier than late 2019.

Do you agree or do you think we'll see another game from them in just 2/3 years? If the one I pictured was the case, do you think it's worth it to wait so long for such amazing games ?

Nope. Xenoblad Chronicles X is set up as the first to a multipart series. I can see them releasing its sequel in 3 years. So 2019 sounds about right.

Also Monolith has two studios, one is meant for helping Nintendo with their games and the other makes their own games.

I don't think it would be wise to do that. If Monolith wants to do another Xenoblade sandbox, they can just reuse all the assets, engine and mechanics and make a new game in three years tops. Nintendo wants to make its software output faster, and for that, reusing some things might be inevitable.

XBX was their first HD game, so it took more development time. Monolith Soft consists of two development studios, one of which was created specifically to aid Nintendo EAD. That's nothing new.

Well, it really depends on many things.First if XCX finds critical and commercial success(in the west), its possible that Nintendo gives more resorces to them, and also more staff, and they may prioritize their games in opose of helping other projects.It also depends on the next game, if its going to be a new entry in XC or if its going to be a new IP.If its a new XC, then theyy could probably use assets from XCX and ideas pre-estabilished in them, making the development much faster.If its a new Ip on the other hand....

Well, in any case, I dont think they will take an extra 4 years to release a new game.They are already working for 8 months in a new game after all.

Well, Monolith Soft has two studios: one in Tokyo and one Kyoto.
The Tokyo-based studio develops new games like Disaster, Xenoblade and Project x Zone. The Kyoto-based studio helps Nintendo with their own games like Skyward Sword, Pikmin 3 or recently Splatoon. So I don't see a reason why this help for Nintendo could affect Monolith's own games.

But of course we won't see the next big game before 2018. 2020 is a bit late.
Maybe we will get a Xenosaga or Baten Kaitos HD Collection in between.

And (obviously) the next big game will come for the next home console.

I figure it'll be a XCX style MMO launched within the first 2-3 years of the NX

Does anyone else remember hearing about a Monolith game in development for 3DS???
I'm not referring the the Project X titles; I remember hearing about an RPG epic...
It's been a few years since it was mentioned...

Nah... studios take 2 years between projects. With a publisher like Nintendo they may delay it for quality, but shouldnt go longer than 3 years.

So, you can expect their next project around 2018. If they made a direct sequal to Xeno X, it could be out sooner than that, if they stick to the Wii U or architecture that uses the same tools.