I'm really loving the modular nature of the 5E D&D system. The great thing about modular systems is the ability to remove or change parts of the system. Sometimes to "fix" a part of the system. Sometimes it's to mold the system to get "the right feel" for a given campaign.
One such part of the 5E D&D system is the proficiency system. It would be very easy to customize it to taste. One could change out skills or even expand or restrict the number of proficient skills based on class, race, or background and not unbalance the game.
I'm also a very big fan of the DMG optional rule of being proficient in some abilities scores rather than skills. Much like my beloved Castles and crusades. Would be just as easy to also add proficiency bonus to any roll that just "makes sense" based on the characters Race, Class, or Background.
If you wanted to run a simplified (even more simple) version of Basic D&D you could ditch skills and just add Proficiency bonus to all rolls and just use that and ability bonuses. giving a +2 bonus to any roll that is covered under the class of the character, Like a rogue picking locks or moving silent.
Some may even like the DMG optional rule of rolling a die rather than use a proficiency bonus. This too has some potential for hacking. For example, rather than adding bonuses for class or other sources you could give a type of advantage/disadvantage by allowing more than one Proficiency die to be rolled. Picking the the best or worst one rolled.
Using the system to run modern of future games would be very easy. After all it's easy for me to treat a computer use as a tool proficiency. Using a car could be as well, after all piloting a boat is in D&D.
There are so many ways to make the system 'your' system. Proficiency is a big part of the over all 5E system. Nothing would add more flavor or customization than hack the proficiency system.