You might consider another level of Rogue, two levels of Beguiler, and then the Unseen Seer (Complete Mage) class. It focuses on Divinations, which would be useful for an investigator, while still giving you lots of skill points for Gather Information, Knowledge, Listen, Search, Sense Motive, and Spot. It gives you a 3/4 BAB progression, so no worse than Rogue there. I'll admit, the d4 hit dice wouldn't be fun for a melee type, but things like mirror image could help you there. If you took all 10 levels of the class, you'd have 6th level spells, your BAB would be solid (+10 at level 15... or +11 if you're allowed to use fractional progessions for BAB), you'd have a respectable sneak attack... could be fun. You can alternatively go with two more levels of Rogue and one more of Beguiler - extra d6 of sneak attack in exchange for dropping to 5th level spells.
It gives you another tactic when hitting things doesn't work. It also lets you do nonlethal damage - the whelm spells were mentioned earlier. Heck, find a merciful spellstoring greatspear, and you should be able to knock out folks left and right.
It gives you another tactic when hitting things doesn't work. It also lets you do nonlethal damage - the whelm spells were mentioned earlier. Heck, find a merciful spellstoring greatspear, and you should be able to knock out folks left and right.