The most obvious candidate is the Surface Book. Microsoft introduced the current base model back in 2015, and the 2016 refresh ultimately amounted to a high-spec option rather than a true replacement. Provided the Surface Book line carries forward, it’s due for new processors (8th-generation Core seems likely), new graphics and other tweaks that you’d expect after two years.
There are other possibilities. That promised LTE Surface Pro has yet to materialize, and the Surface Studio all-in-one is nearly a year old (with graphics that were outdated when the system was new, we’d add). The Surface Hub is long in the tooth, and there’s always the chance that Microsoft will unveil a new form factor just to keep people on their toes. In short: while there are a few front runners for hardware updates, it’s hard to completely rule out surprises.