I'm a fan of the black powder mauser rifles myself, and they can be really reasonably priced. A Gew71/84 would be a good addition, although if you want to shoot you're really going to reload unless you're made of money.
Grew98 is kind of a must-have if you're building out a general Mauser collection.
I'd argue that a Gew88, while not a mauser, is a good companion piece to show what the competition was. It also pairs nicely with a Belgian m1889, because you can see what the contemporary Mauser was like and some of the similarities they had between them - the barrel shroud for example.
Argentine m91s are plentiful and generally in good condition. Very pretty guns, unfortunately usually with scrubbed crests. Still a great window into the kind of quality that DWM was making pre-WW1.
The Spanish m95 is another must have, especially if you're considering the 1903 a Mauser derivative.
Brazilian m1908s are cool guns, basically a modernized Gew98, and can frequently be found in excellent condition.