Any of you ever played God Hand? No? Oh...
Well, the way I'd expect modular melee stances to work is as follows:
You go up to some NPC like, I dunno, 'Swordmaster Chuck.' Maybe with a design made to make you go 'psh, my warframe looks way cooler' just like Teshin's. He or she would offer to give you a template stance mod in exchange for rep points from some arbitrary syndicate like Onkko's or LD's. When you pick what weapon type the stance is for, you'd then go into a menu where you get to see a list of different attack animations relevant to your weapon of choice. They'd all have different names and come with a little 'gif' kinda like when you check your frame's abilities. Each of these attacks would cost a set amount of, let's call 'em "stab points."
You'd have a limit of 30 stab points or so, and you'd spend them on both attacks and properties. Say you're making a rapier stance and you put in a regular poke as the first attack of your 'EEEE' combo. You could spend 4 points to make that combo just the same boring jab repeated four times, or you could mix it up with some swipes and twirls and dabs and what have you.
You'd get to choose each and every attack for up to four different combos, as well as designating their inputs. The stronger the attack is, the more points it costs. This would be to prevent people from composing all their combos of the same 13-hit attack that procs slash on every hit. Instead you'd make your stance like you'd make a build: You decide whether it's dependable and versatile, or if it's limited but specifically designed to excel on one or two things.
I think letting people make their own melee combos up - especially with these recent melee changes - would make them consider maybe trying some weapons they never had an interest in BECAUSE of their moveset. Personally I never liked the polearm, staff, machete, fist, or war fan stances. I think they all have certain attacks that are fun or interesting, but come with others that are annoying and restrictive. If I could replace or remove those problematic attacks, I'd definitely give those kinds of weapons a second chance.