I successfully created some fully customized themes and modules for some customers. I ended up taking the Classic theme (beceause we are guaranted that it is up to date) and stripping files, testing each time that prestashop still works properly.
I also used the Prestashop's source code to figure out which tpl were necessary (because sometimes it's not that straightforward).
You can get rid of a lot of tpl file. It took few days to actually have something really minimal to build on.
Here is the list of template file we are using in one of our projects, it might help you (we merged a lot a tpl files that were split in multiple files) :
.
./catalog
./catalog/listing
./catalog/listing/product-list.tpl
./catalog/product.tpl
./catalog/_partials
./catalog/_partials/miniatures
./catalog/_partials/miniatures/product.tpl
./checkout
./checkout/cart-empty.tpl
./checkout/cart.tpl
./checkout/checkout-process.tpl
./checkout/checkout.tpl
./checkout/order-confirmation.tpl
./checkout/_partials
./checkout/_partials/address-form.tpl
./checkout/_partials/address-selector-block.tpl
./checkout/_partials/customer-form.tpl
./checkout/_partials/steps
./checkout/_partials/steps/addresses.tpl
./checkout/_partials/steps/payment.tpl
./checkout/_partials/steps/personal-information.tpl
./checkout/_partials/steps/shipping.tpl
./checkout/_partials/steps/unreachable.tpl
./cms
./cms/category.tpl
./cms/page.tpl
./cms/sitemap.tpl
./cms/stores.tpl
./contact.tpl
./customer
./customer/address.tpl
./customer/addresses.tpl
./customer/authentication.tpl
./customer/guest-login.tpl
./customer/guest-tracking.tpl
./customer/history.tpl
./customer/identity.tpl
./customer/my-account.tpl
./customer/order-detail.tpl
./customer/order-follow.tpl
./customer/order-return.tpl
./customer/order-slip.tpl
./customer/registration.tpl
./customer/_partials
./customer/_partials/address-form.tpl
./customer/_partials/customer-form.tpl
./customer/_partials/login-form.tpl
./errors
./errors/404.tpl
./errors/forbidden.tpl
./errors/maintenance.tpl
./index.tpl
./layouts
./layouts/layout.tpl
./page.tpl
./_partials
./_partials/form-errors.tpl
./_partials/form-fields.tpl
./_partials/subcategory_list.tpl
There might still be some room for simplification, especially on the "customer" part.