You have to look out for all the usual things, bonus, royalty, post-production costs, indemnification, warranty of title, shut-in payments, Pugh clause, formation limits, the works. What you negotiate for is somewhat dependent on how large an interest you own. If you have all the mineral rights plus the surface on a large tract you're a lot more likely to get significant changes than if you own 0.2 net mineral acres in a 32 acre tract.
Where you know the broker will be flipping the lease to another company you can expect not to get quite as much in the way of bonus and royalty amounts as the broker may be making money on the difference between what it pays you and what the company pays it. I'd be pushing for at least $4K/acre and 18% gross proceeds in Marshall County. You might be able to get quite a bit more for the bonus, and maybe more for the royalty depending on how large your interest is.
Also in this specific situation you may not be able to get the broker to be as flexible on some of the terms. It depends on how much communication there is between the broker and the company. The company may have given the broker very clear lines which the broker is not allowed to cross, or the company may be working very closely with the broker to personalize the leases. It's hard to say until you get into negotiations a ways.