Zardnaar
Legend
I think you'd be better off starting with 5E as the core and building out from there. Stripping that core down a bit, streamlining some things, bringing back more explicit frameworks like skill challenges (only better designed and playtested), then modding from there to suit anything from AD&D to 4E would be a snap. None of them would be 100% as-originally-printed accurate, but you could play all of them on the same chassis. Most of the differences are thematic and focus. Focus on resource management vs handwaving. Focus on winning the fight before rolling initiative vs after. The rest are mostly minor mechanical differences. Higher vs lower hit dice. Higher vs lower ability modifiers. For a more old-school game, use these optional rules. For a more tactical combat-focused game, use these optional rules. Etc. You know, the promised modularity of the 5E playtest.
I thought saga etc had some advantages over 5E design and if you're using feats I prefer micro ones.
I would overhaul the feat design to be more inclined with 5E.
But yeah you could also use the guts of 5E (skill and proficiency) and use ot to duplicate SWSE, 3.5 or B/X I suppose. You could add outright encounter powers or whatever into it.