If you're in the property for the medium/long term, then do what makes you and your family happy . Prospective buyers will realise they can rework the room to suit them - it won't be the reason they don't buy your property!
I agree with robinm’s advice- if you are planning to stay in your home for the long term, then you should design your new bathroom to suit your needs. When you decide to sell, your agent can advise you and, depending on the state of the market, you could refurbish your bathroom at that time to attract buyers by replacing your shower with a shower/bath. Depending on the configuration of your bathroom (and any plumbing issues) you could install a bathtub with a shower above it relatively cheaply- a fairly decent bathtub costs around £100ish.
We regularly renovate bathrooms and whether or not a client installs a bath or shower depends on a number of factors- the type of property and number of bathrooms they have, their needs and whether they plan to sell or let the property in the short term. I think that not having a bathtub is more of an issue if you let a family home (because families with small children can be put off renting a home without a bathtub) but it is less of an issue if you are planning to sell because buyers tend to renovate kitchens and bathrooms after they move in to suit their own needs and taste.