thematic planning is one of the most fun parts of an otherwise process that can feel like a bugger (recital prep is not always enjoyable!

)
+1 to above.
it may open new possibilities to broaden the theme, or have a dual theme, perhaps a more obvious one and a slightly more subtle one (i once grouped pieces that were related by tritones as tonic distances from each, and used an opening that featured a tritone as a prominent part of the melody, one or two attendees pulled me aside after the performance to remark if i planned it that way

).
so first define the technical range and limit of difficulty, then look at your tastes and what you would like to express. you can do simple common era, or 'war time music' or nationalism (iei composes of same country) or by form, or forgotten masters, etc. it's all pretty open ended unless you are bound to formal requirements by a degree program, competition rule, or recital committee
good luck!
