I think that Minecraft should add at least another dimension because I think that seeing either a reverse world where the villagers are the enemy and the mobs are your friends would be cool, as well as new tools such as scythes for harvesting crops and a crossbow to add another ranged weapon, also some new ores would be nice just to be there maybe even giving them a secondary purpose, such as maybe a diamond pickaxe with ruby extension to make it more efficient. I don't know this just sounded cool in my head.
Also new mobs would be good like Vampires, and Basilisk, or just snakes in general, they added Turtles add some freaking snakes to.
Also add a role system for multi player games, like when you come into the world there is a fighter, a builder, a gatherer, stuff like that.
Vehicles such as maybe a carriage for horses and a warship would be nice. Bringing back some of the old stuff such as the shields, quivers and backpacks would be nice.
Also just talk to moders about what they did, moders have a lot of cool things in the game, talk to the moders pay them for their packs and make them updates, then have them join the team

Simply put, Mojang hasn't added any new dimensions just yet for a reason. People have been asking for this for a long time, and Jeb has stated repeatedly that he wants to work on making the current dimensions more interesting and enjoyable before they would add a new dimension. As to the ruby armor, I have seen far too many people asking about that without considering why it doesn't exist.
The current armor grades exist as they are for the purpose of balance. The normal gradient is leather > chain > iron > diamond, and as a balancing element, gold. While diamond is easily the most defensively powerful, gold is the most enchantable, easily grabbing you Protection IV with a level 15 enchantment. Were you to add Emerald armor, or any of the other various gemstone armors that seem to crop up so often, you would have to go through the process of figuring out just what the proper place for it would be in the resource scale. As-is, the most likely addition is Obsidian armor and tools, which would be comparable to diamond in effectiveness, or perhaps slightly less, and would have much higher durability. Perhaps they would add a slightly lower enchantability rating to further balance it. I state as a disclaimer, however, that there likely won't be the addition of Obsidian items in Vanilla for some time yet.
As a counter, I will remind everyone reading this that it has been confirmed there will be new ores in 1.8. While Mojang hasn't yet revealed what exactly these ores will be, there's a strong likelihood of there being at least one new metal, a new "secondary purpose" mineral, such as Redstone or Lapis Lazuli, likely to act as a rarer supplement for the new enchanting system. And as an even wider bit of speculation, being that Mojang is wont to hint at their next update in the current one, perhaps they will add Rubies to hint at a new trading system in their next update, which could be an expansion on the Nether. Pigmen, anyone?

I think they should lay off from adding new things for now. I believe they should think about optimizing Minecraft. By that I mean, making it less laggy for the people who don't have the greatest PC.

No, I don't think they should.