Has there ever been a composer between Baroque and early Romantic periods (within piano literature) who created a new form based on a combination of previously separate forms into a new iteration or manifestation?
I'd argue yes with sonata rondo form. Sonata rondo form combines both the tendency to go to a related key and back of sonata-allegro form with the persistent repeats of the first theme of rondo form. Perhaps the simplest version of sonata rondo form is A-B-A-C-A-B', with B in a related key (e.g. dominant, relative major) and B' in the home key.
According to the current Wikipedia article on sonata rondo form, the source "Girdlestone, Cuthbert Morton (1964) [1939, 1958]. Mozart and his Piano Concertos (Republication of Second Edition). Mineola, New York: Dover Publications. pp. 48–55. ISBN 0-486-21271-8." argues that sonata rondo form may even be an innovation of Mozart's.
Sonata rondo form is quite common in Classical-era music, not just from Mozart, so that form either presumably spread fast or was independently reinvented. Regardless, I don't think Baroque music uses sonata rondo form (comment below if I'm wrong!), so there's your answer.
Also here it is to say - as else where:
Theory is following and reflecting the practice!
The classical forms are typical “classical” abstractions. In reality we’ll find a lot of mixtures of all elements. The pure form could be the exception while the combination of all elements of the “Lied form” (adagio), Scherzo, Menuette, sonata, variation could be the regular form.
(Symphonies, Sonatinas, Concertos, Sonatas etc.)
Similar to @Dekkadeci's answer you can find sonata type movements where one of the themes is a simple binary form. These cases can be very obvious where repeat signs in the score clearly delineate the binary structure.
I seems to notice this in Mozart piano sonatas in slow movements.
I don't think anyone would call such forms 'new musical forms.' But they would exhibit one of the characteristic you describe: a combination of forms. This being specifically a smaller form within a larger form.
(I will look for a specific Mozart quotation and update this post later today.)
You probably could find examples of theme and variation form within a larger form. But it could be tricky to make a case of that being the one form inside the other, because classical style can be summarized in part as repetition with variation. In a way variations inside larger forms is happening all the time.
Fugue is often described as a process rather than a form. Nevertheless you will often find fugal passages inside large movements and that is an interest combination of methods.