I had an initial look for potentially good DM replacements in the following link, but TLDR there weren't many:
https://www.redcafe.net/threads/centre-midfielder-transfer-links.488776/post-33997872
I thought I'd have another go at doing a similar thing, but tweaking slightly to focus a bit more on stylistically similar players to Casemiro. I used his current season's data as a reference as it's still excellent and well beyond most other midfielders out there.
Mainly, I filtered out players based on a number of Casemiro's key stand-out, differentiating attributes; primarily: high defensive work-rate, strong progressive passing, and not being bad in the air. If restricted to more additional data-points eg only exceptional in the air, plus scoring goals, there'd be basically no one left.
I decided to add Sofascore's season player ratings for wider context, because some of the extra bits-and-pieces Casemiro does relative to other stylistically similar central midfielders (eg goals, headers etc.) will still get captured there to some degree without narrowing the field too arbitrarily. Looking at Sofascore's ratings relative to other data sources and my own observations, they seem fairly solid for assessing player performance.
Younger Prospects from Smaller (non-Big 5) Leagues (age, club, Sofascore Season rating):
Vanja Dragojevic (20, Partizan Belgrade 7.02); Mathias Siltanen (18, Djugardens, 7.15); Soumaila Diabate (21, RB Salzburg, 6.79); Aleksandr Stankovic (20, Club Bruges/Inter Milan, 7.32); Kamiel Van De Perre (21, Union St Gilloise, 6.91); Justin Janssen (19, Nordsjaelland, 6.91); Cauan Barros (21, Vasco De Gama, 7.07); Shea Charles (22, Southampton, 6.93); Stije Resink (22, Groningen, 7.23); Youri Regeer (22, Ajax, 7.09)