Distance gauge on your mouse. So, for example, if you use ctrl to lock-on to a target in the distance, it'd give you a little measurement for how far it is. I think it'd be interesting, and it seems like it'd definitely be a challenge, if that's what you're looking for.
In a recent video posted by someone of a foreign server (detailed, I know), I remember seeing these words on the ground in Iria. Apparently, they removed whatever hides artifacts before they are "discovered," so you'd see these little words on the sand next to Filia before they used an l-rod in that exact spot, and the word would disappear and the discovered object rendered along with its name right above it (Eye of the Sun, in this case.)
Otherwise, can't think of anything non-gamebreaking. There is this idea I'd like to mention though, the aim meter got modified for elves if you used that patch provided by Aldeel that changed the animation.
For example, normally, if I'm aiming and the aim meter hits 99%, but then I move, it jumps to 50% and begins aiming from there upwards again. Now, if I start running away from something and I'm moving, I'll see the aim meter at 50%, but the second I stop moving (if I've been running for long enough) the aim meter immediately goes to 99%. Now, this is only if I've been running and "aiming" long enough for the meter to be at 99% without me moving.
Like, say I start aiming. 10%, 20%, okay I move. Stop. It's still going up, 40%... I move again, and the aim meter caps at 50% since I'm moving. The second I stop moving, it's at 60%. 70%. 80%. Move, it caps at 50%, but the "second aim meter" continues rising and the immediate moment I stop moving, it jumps to whatever it's at. So, I stop moving, it goes immediately from 50% to 90% (it was 80% before I started moving.) Does that make sense?
Well, the proposition is you experiment with calls in standard.dll, and see if something interesting happens to human range, or otherwise.
Good luck~