Xill is also underwhelming compared to Thri-Kreen but the Xill starts with a point of BAB which means I can actually take combat feats at first level.
+1 BAB monsters, eh?
Feral Gargun + Ice Beast would make a nice Yeti. (+1 Str/+1 Con, Powerful Build)
Gargoyle grants a decent way to go unnoticed and some interesting attack options, plus +2 Str/Con (though -2 Int)
Titan also gets powerful build, some SLA's, change shape (though no stat mods at 1st level).
Your
Marrusault also has +1 BAB at first level, d10 HD, and 4 skill points/lvl. Not an awful base to build off of.
Flind Gnoll also gets 4 skill points/lvl, free disarm attempt on critical hit. +2 Str at first level.
Obviously there are plenty more options, depending on the overall concept of your retired boss monster; These are all generally humanoid in shape.