It's already a fps :p

If there was a Minecraft 2 then i believe there will be more of an Adventure mode, while the 1st one offered some adventure the sequal might add more dungeons and increase the world to achieve epic boss fights with an all new art style. Minecraft 2 can be done and be successful, it just needs a careful approach to do things the original cant do with simple updates and that would be hard.

Merchandise is also massive with MC. Only releasing DLC will ensure a slow death of the franchise and the merchandise. I don't think that's what MS want. They will release stuff to try and keep it relevant and see a return on their investment. They probably outlined the plan before the takeover but what they have planned is anybodies guess.

By not making a minecraft 2

This actually sounds viable to me.

a 3D adventure style / dungeon crawler minecraft could work.

Minecraft 2 will never happen and if it does it will most likely flop terribly.

Minecraft 2, is Something Useless, i think that Purchase Minecraft, for 2.5 Billions was too Much Money, they would be able to make over 40 Halo games with that Money.

Making a Sequel to Minecraft, is like Making a Sequel for Pong, there is Really Nothing that can be Done, If they Want to Change the Things so there was no Need to buy the Franchise, the Only Need was to make a Game like Minecraft, just like Square, is Doing with Dragon Quest's Minecraft.

Having official modes or the likes (there are tons of mods that do this though)

Releasing more updates is probably the way to go now, as making a new Minecraft may not be that widely accepted

Honestly I like the idea, of building things, and have villagers move in.

Then have them ask you to build x,y,z for you, which in turn requires you to be able to do a quest, for kill & collect materials.

Leveling systems, combat systems, team work (with other builders) to take down big foes (dragons?) for items needed to build certain things for the villiagers.

It puts a RPG spin on minecraft.

Azzanation 's idea of a 3D adventure / dungeon crawler could easily work too. 

From what Ive seen of lego worlds, it looks like it went abit in that direction.

ANY game can have a sequel. Will it be any good? We won't know until is comes out.

I am sure there will be a sequel, but probably not for 2 more years. No need to do it with it continuing to sell like it is.