Well I have a tutorial on Buttons and one on an arrow selector, which you should be able to gleen some of the process from. Eventually more form elements will be added. In general most UI elements should be buttons (for evidence of this just look at most AA and AAA games). Drop downs, tho I have used them in UI’s, are not recommended for end user product UI’s (other than map editors), but they can be helpful for other screens like character creation, etc…
My tutorials as of now are more based on building game’s, and are mostly focused on the basics as most of the specifics should be filled out by the individual game as every game has its own needs. I think focusing on UI elements will would not go over well with my subscribers, though adding them over time is fine and will happen.
Monogame is more than capable of doing whatever you can dream up, but it is not really meant to be an engine like Unity (which contains more of the tools you are mentioning). Monogame has the ability to create complex UI’s you just have to code the perfect solutions for them which is the beautiful part of Monogame.
Keep in mind that every indie that makes games is doing something that is usually developed by a team of devs 
Tooling is one of the most important things a developer can do, so I agree that these elements are helpful, but in order to move forward with Monogame you may have to spend time developing the ones you need most when they are not available. For instance the Arrow Selector took about 1-1.5 hours to make, that time is easily recouped as soon as you use it a few times. A first version of Drop Downs should probably take about 2-3 hours of work provided you know what you want them to do.
TLDR;
More will come over time, but if it is to slow you may need to build the ones you need most…