I am not in love with the bevy of immunities that emerge in a high level game. If you wanted to venture into house rule territory, I'd prefer to attack the immunities at their source rather than come up with a work around. This has been brought up in this thread already, but I'd just put my vote in that camp.
At any rate, Brute Force Spell strikes me as wonky. First, it seems overpowered for a +1 adjustment. I know the comparison is to Searing Spell, but it's relatively easy to get either fire immunity or a ton of fire resistance. Other immunities are not so common. And, letting some damage burn through an immunity has less impact than suddenly making a target vulnerable to a whole range of effects, many of which are save-or-suck.
Second, and a bit contrariwise, "not against enemies 'immune to it by nature'" clause seems problematic to me. This feat then just seems to resolve to: "screw you Mind Blank and True Seeing!" Tis a noble goal, but this seems like a weird way to go about it. And, if I was a player in such a game it'd smell fishy to me. The undead and what have you keep all their immunities, but the only ones we have ready access to vanish. This doesn't exactly address the problem of making the Illusionist and the Enchanter viable again; it only does it on one side of the screen.