I actually prefer the '502 and that was my first real EDC for ages. 1-mode, XP-L 5000K or so.
Prefer lower K, like 4000K or so, so if I had to redo it, that’s what I’d stick in there, maybe a ~4000K 519A for nice CRI and tint.
My '501s are mainly hosts, but I have a purple '501 host with 365nm UV drop-in and ZWB2 filter.
No pix of any of them, as I just, well, use them…
I really despise those retarded thumb-cutouts on tailswitches, which is why I always liked the '501 and '502. Especially the '502 with the smooth rounded tailcap. Just a nice feel.
Whether the button’s flush, proud, or recessed, NO cutouts. That would be a dealbreaker, especially if I can just go back to any of my existing hosts or complete lights.
I’m also not a fan of clips, and almost immediately rip them off of any new lights I get. And again the '502 is an exception, with the screwed-in clip that actually has a nice contour, and instead of just digging into my hand like most other clips, actually lets me get a decent grip on it, so I don’t mind it (even though I never use it).
If I were to use either as a “tactical light”, I’d have a forward-clicky switch (ie, momentary-on) to be able to flash’n’dash. Also, 1-mode only, as to not start switching modes, brightnesses, etc., by just flicking the switch too quickly or accidentally touching it in a backpack, bag, pocket, etc.
For a multi-mode EDC, reverse-clicky is probably better, but low-to-high and NOT high-to-low. Also, no blinkies in the main cycle, better to just hide them well out of the way, like triple-click, and double-click for “turbo”.
A sucky UI, and I just won’t use the light, as there are plenty of lights with UIs that don’t suck. Too many great lights in hardware are completely ruined with a retarded UI in software. (Looking at you, Wuben…)
So… the '501 and '502 are already pretty damned good as hosts, and the problem is the drop-ins they often come with. I wouldn’t change the hosts at all.
You can change the beams/uses by, say, a large-chip emitter with orange-peel reflector for a “floody” beam, small chip emitter with smooth reflector for a tighter throwier beam, etc. Take care of any artifacts like rings, corona, etc.
Maybe offer inserts of diffusion film to fit between reflector and front glass. Lightly-frosted film can diffuse the beam slightly to simply eliminate some artifacts, or heavily-frosted film can cure a multitude of sins.
Just some ideas. Go crazy…
Alright, since participation numbers are lower than expected, I’m opening up the competition to all UltraFire flashlight owners – let’s get more folks in on the action! 
