I am a big fan of both the original Alternity and d20 Modern/Future. I am curious what experiences folks have with various modern and future iterations of the 5E engine, such as Ultramodern5e, Esper Genesis and others.
I am one of those multitudes that start playing in the 80s. I have played more systems than I can count, and I have plenty of non-D&D inspired games in my repertoire. But it is often useful to have a transition or adjacent game for folks that are not inclined to learn new game rules. Whatever its other faults, d20 Modern/future was really good for this, as were the various d20 Star Wars games and Mutants and Masterminds.