It's an interesting assignment. To some extent, your travel policy needs to be at least competitive with similar companies in your industry. OTOH, a more generous policy or one that gives the employees more control can result in business travelers who perform better and are more loyal to the company.

Do people book their own travel, pay for it with their own credit cards, and then file for reimbursement? Do you have corporate travel contracts or preferred providers for airlines/hotels/rental cars?

Will the company pay for lounge memberships, seat fees, checked bag fees, etc.?

Are employees required to travel at inconvenient times or connect to save money?

Do you reimburse actual expenses or per diem? Under what conditions is alcohol reimbursed? If actual, any guidelines on bell person tips, valet parking tips, laundry and dry cleaning expenses?

I assume that the company self insures for additional expenses during IROPs and provides health coverage, including emergency air evcuation, if foreign travel is required.

Do you encourage car service to/from airport, taxi, Uber etc., public transportation, or driving? If driving, any rules about airport parking depending on the length of trip?

Can or should emloyees spend an extra day or two at the destination in order to save substantially on airfare? How about arriving a bit early to be well rested before major meetings?

ADDED: I suspect that your travel policy needs to be in compliance with the tax code, not only from the viewpoint of being to deduct employees' travel expenses from income for your corporate (or whatever) income tax but also so that you don't need to track and report reimbursements to employees that the IRS considers to be income. [Fortunately most state tax codes are similar to the federal rules.] If you're a publicly held company, you also want to be in compliance with reasonable accounting standards. You might eventually want to have a lawyer and an accountant look over the proposed policy.