OP is just looking for examples from the community's perspective
By yelling at them to simply "PLAY MORE" you're effectively silencing any potential for a discussion to arise
Anyway
An endgame map to me should firstly feel like an endgame. I don't want to see a random level 14 mercenary running around, nor do I want to see iron lances. On the opposite side, I do not want to see a bunch of 20/20 units with all stats capped. There should be something going on on the map, not just "run and kill the boss". FE5 has the dreadlords for example, and FE16 VW
Some kind of subboss adds a nice element.
There are other things, such as dynamic events, like when fighting any of the final bosses in fe10, they have an AOE effect every so often.
It should feel like a rough fight to get to the end, but it shouldn't make me sit in place for several turns fending off infinite reinforcements. Surprises like traps, map changes, weather conditions, hidden rooms (think fe7), things like that. Not ambush spawns.
As for desert maps, the problem with them to me is they are too large most of the time and you are limited to moving 1-2 tiles per turn. Mixing in different tiles, such as plains, so you have sections that are faster than others helps alleviate the sluggish feel, as well as gives the player more options than just fliers/magi(if magi can sand walk). I personally have done a project that featured... 4? Desert maps within 6 chapters, and I received little complaints if any to my knowledge. Keeping them smaller, giving the player options, and either having a way to ensure the player can find the hidden desert item or explicitly make clear there are not any.
Those are just some thoughts