I'm not going to blab on about random stuff so:

1. Terrain

when making a map, use a custom terrain generator such as WorldPainter

Try not to use villages as a town and try to design your own buildings

if most of your map is based inside, then a normal flatworld would be easiest

2. Items

Notice that in Hypixel's adventure maps, 99.99% of the time all the items have names and/or custom enchants

You can get these by using anvils (Boring) or using a third party program like NBTexplorerer or mcedit

There are some very good filters and guides on the web

3. Story and Genre

All adventure maps need a story, so try and think of one before creating the map, and sculpt the landscape and buildings to match it

What is your Genre? There are many different genres to choose from. like the storyline, pick a genre and build around it.

4. Bosses

Every map needs a final or miniboss throughout the map

Some good pointers are

Use mcedit to create; spawners, geared mobs, boss items.

Create a boss room. some features may include; large open area, boss minions, and lots of redstone (Watch some map maker tutorials and redstone geniuses to get some ideas)

Well that is all I can think of for now so please reply in kind and tell me if this helped