Not a fan of this idea. In addition to the 5e system being incredibly plain, making it a module makes it feel very subordinate to 5e. As someone that has mainly used the PF2 system in Foundry, there's so much room for better. New system in the Foundry system list gets a LOT more eyes on Level Up as well, way more than it being a "5e mod module". Plus, by default the 5e system is only the SRD anyway, so anything there could be incorporated into the Level Up system as modularity options anyway.
If there is a Level Up system that supports using 5e rules as well as content (like standard rules for Expertise, etc), and can use imported 5e material, that would be good at a basic level.
The issue is that it won‘t be compatible with 5e mods, and while Foundry is solid and functional without them, there are design quirks (like the default overly long sequence involved in making a roll) that make most people want to use various quality of life mods. And beyond that, there are a lot of cool value added mods, and the ability to use such customization is one of Foundry’s major appeals. Now, once Level Up becomes popular enough you could hope that the designers of some of those mods would make a version for the Level Up system, or reach out to them, and that would probably get some elements of the functionality in. The Level Up system could also include popular functionality that you need mods for in the 5e system. But as its own system it’s never going to have all the same mods, and it means you are going to make people have to decide whether they want to use the Level Up system or keep the mods they like.
Ideally, it could be a very robust mod added onto 5e with a lot of options, so you can play with all Level Up features or just selectively activate some of them, while maintaining compatibility with 5e mods. If that isn’t feasible, an alternative could be to have both a Level Up system for if you don’t need 5e content or rules used, and a mod to add on to 5e system if you prefer to use it for rules or other mod compatibility.
Since it will be a while before they get to it, the Foundry 5e system will be more mature by then and they should be able to better examine which considerations are most important and how best to meet them.
