There are two parts to maternity leave: ordinary maternity leave (OML), which refers to the first 26 weeks of leave, and additional maternity leave (AML), which refers to weeks 27-52. You have the right to return to exactly the same job if you return during OML. If you return during or at the end of AML, you have the right to return to the same job unless it is not 'reasonably practicable' to do so, in which case, your employer must offer you a 'suitable alternative job'.
If this is your second or subsequent maternity, parental or adoption leave, you might recall from last time having ideas about your ideal work scenario and then changing your mind closer to your return date. However, if there are aspects of your job that you can envisage being quite certain about wanting to change (for example, amount and type of travelling, hours worked, location of work) it's worth flagging those with your line manager before you leave. A good reason for doing this is to allow your preferences to be incorporated into any changes that may be discussed or implemented whilst you're away (that don't legally require you to be consulted on). If you agree to work shorter hours it may be that your ability to pitch for tenders, go to networking events, client events etc is diminished. If that is the case, some of the metrics for remuneration and promotion may suffer.
Many employees who are taking maternity, adoption or parental leave anticipate wanting to return to a fresh challenge, be that a promotion or a sideways stretch. If this is you, talking about aspirations, strengths, skills you'd like to build and experiences you're keen to have is a really useful conversation to have with your line manager. As above, it builds a picture in his or her mind that may be relevant when thinking about changes within the team.
Your ability to reshape your role when you return may be affected by how well the person or people you've handed over to have performed. Whilst your ego might not want to hear how terrific things have been in your absence, it does pave the way for you not to have to come back and slot into the 'same old, same old'. Equally, you can claim credit for setting the team up for success and use that to your advantage later when making suggestions for role changes.