I believe that Renegade ECW are much larger than GW offerings, but I could be wrong on that. In any event, if you simply want to replace "Empire" troops with various Renaissance figures (earlier than ECW), then there are many offerings. I'm not sure how "big" you want your figures to be, but most ranges are 28mm size, which roughly corresponds to GW.
For starters, there is Foundry. A bit pricey, but there are some real gems in their line:
Landsknechts:
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Gendarmes:
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However, my favorite gendarmes are made by Eureka:
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Gallery picture here:
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Here is a comparison of Gendarmes from various manufacturers and GW:
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More comparisons of Gendarmes:
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Comparisons of Landsknechts (the rough equivalent of GW Empire troops):
Pikes:
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Arquebusiers:
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Then there are The Assault Group (TAG) miniatures which are superb. The range is young, but already has many offerings from the period of the landsknechts ("Empire Troops")--they do not yet make landsknechts or their arch rivals the Swiss, but hey are probably coming out next year. So far, they have:
Italians:
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and Spanish:
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Old Glory also offer quite a few figure types for the period:
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And, Artizan have recently released Landsknechts, which are quite nice:
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Comparison photos showing Artizan landsknechts side by side with other manufacturers:
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And, a GW Empire spearman (not sure which edition--6th??) matched up against an Artizan landsknecht:
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Redoubt (as mentioned in the prior post) has many offerings, but they are a little "hit or miss" in my opinion. I have many figures from their Renaissance range, and while there are quite a few that I like, many are average (probably quite nice "back in the day.")
Eureka through their 100 Club are gearing up to produce early 16th century Swiss infantry and Italian infantry. As I mentioned earlier, their gendarmes (and more recently released lesser gendarmes--or archers) are superb.
So, to recap
there is a wide range of figures from many manufacturers--offering a tremendous amount of choice for the period.