The issue with heavier spicing in the prep is that for a loin, it won’t penetrate the meat like a marinade would for a whole leg of lamb.
I’d also want to make sure the spicing doesn’t taste “raw” in the end. You could brown the spice-coated outside at the outset or at the end, to safeguard against that.
The flavor profile from Indian whole raan (lamb leg / shoulder) would work nicely — Sikandari is the most famous, but there are others.
Much simpler and also delicious, a mustard and herb crust. A friend made this with racks last thanksgiving, and it was fabulous. Lots of mustard, generous garlic, herbs of your choice (swap rosemary for thyme or HDP), and optional fresh breadcrumbs (roasted) or panko.
ETA: it just struck me that you can get a stronger marinade or spices better distributed in a loin by butterflying it & then rolling & tying it up.
My aunt does this with a beef roast at Christmas (garlic, parsley, anchovies, etc) and it turns out pretty spectacular and really well flavored throughout. And it’s not as fussy as it sounds — I even did it once when she hurt her hand 
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