Probably not the first one who have thought of that (wasn’t able to find any relevant existing discussion though), so here it goes. Originally wanted to suggest it as a mod, but not sure about copyright issues etc. (Also I got an idea long time ago when there were no Definitive Editions.)
I think that problem with first AoE (including DE) is that is not so polished & balanced as a second one - no gates etc. Honestly I never really got that much into playing it just because I got to know Age of Kings first.
But what I really dig is the era first one takes a place in and I thought it would be really great to have its campaigns & civilizations available as an addon/DLC for AoE2DE. Probably a separate one (no mixing of civs from AoE1 with AoE2 on same map as they are very asymetric), but with some additions from AoE2 (mentioned gates, formations, some extra units for better game-balance etc.) significantly improving (and changing) its gameplay.
Since most of the units are already done for 4K, it shouldn’t be that much of an extra work and devs could still charge some money (with a big discount for owners of both games).
What do you think?