I recently posted a scenario presenting different QoL suggestions: Age of Salamanders (presentation here , and Mod available here ), mainly to show the interest of more optimal walling solutions.
Other QoL suggestions are also introduced in this scenario, like the Tile Indicator (see video below).
Like for the Optimal Walling, I built a CodePen to quickly test this idea.
For the optimal experience, open it in full page.
Below is an example of how it could look like in the game.
Imagine you want to detect if there is a hole in this wall (configuration from SOTL video).
You start by moving your mouse close to the wall and you press a specific hotkey.
First, tiles occupied by water appear in blue, tiles occupied by trees appear in green (I only did it for the baobab in the bottom of the image below) and non-crossable tiles occupied by buildings appear in yellow (only done for a few buildings in the image below).
This feature is inspired by this mode (except that it would only appear when pressing a specific hotkey, thus keeping the beautiful graphics of AoE2 DE for most of the game).
On top of that, if the tile beneath the cursor is not occupied, it is colored in red. Then, a Dijkstra algorithm is used to propagate this red color to non-occupied adjacent tiles in the vicinity (limited to a distance of 9 tiles in the images below). The red color added on each of these tiles has an opacity dependent on the distance a unit should walk from the cursor to this tile.
In the image below, we see that there is no hole close to the mouse cursor.
But when we move the cursor a bit to the right, we can see that two holes appear (note that one of them can already be seen with the building yellow tiles).
Among all the QoL suggestions of Age of Salamanders, this one will probably be the most controversial because it would make it way easier to avoid holes in walls, which can be a positive or a negative thing depending on your opinion.
Holes in walls are mainly due to the use of a 2D isometric engine for the game and can be partially avoided with appropriate mods (grid mod , small trees, useless plants remover, short walls, simplistic terrain colors with sharp edges …), but at the cost of aesthetic downgrade (this is subjective of course, see SOTL video).
One of the purposes of the Tile Indicator is to keep the beautiful original graphics of AoE2 DE (my subjective opinion 
), and to only alter them when needed (by pressing a dedicated hotkey).
Having said that, the Tile Indicator would make it way easier to spot holes than any of these mods. @Yorok0 mentioned in this post that players having holes in their walls provides great surprises and fun in games. In fact, I kind of agree with this statement and I am not sure myself if I would like to have it in multiplayer games 
.
An alternative would be to only have access to it in single player (campaign missions and single player matches) as was recently done with the sheep scouting.
I think it could also be interesting to have access to it when spectating matches (potentially through Capture Age so that T90 could easily check if there are holes in walls when commentating 
).
So, what do you think about it ? As this QoL can be a bit controversial, please be nice in the comments. The purpose of this post is not to impose anything. I made the Age of Salamanders scenario and this CodePen script specifically so that you can test it yourself and make your own opinion .
It would be way-way too easy to avoid holes in walls with it though.