Here are some thoughts and questions which have come to mind recently. I will definitely be following up on some of them.
• Human communication is not what it seems. This is a fact which typically takes some time (and multiple relationship failures) for us to learn. Even relatively straightforward and sincere messages are routinely construed by recipients quite differently from how senders conceive them such that, while what is being sent and received at the level of symbols is the same thing (i.e. the same set of symbols), what is being sent and received at the level of interpreted symbols are different things.
• The contingent (and unrepeatable) features of any individual's upbringing – which includes as a central element a unique and ever-changing cultural matrix – raises awkward questions about values. We like to think of our core values as being, if not objective or universal, then at least as having some permanent or abiding relevance. But do they?
• Terms like 'moral' and 'ethical' refer to important aspects of human behaviour but I am inclined to think that ethics can only be usefully intellectualized when approached in a more or less descriptive way. Nietzsche in his more scientific moods is my model on this front. I don't know that normative ethics can ever be a coherent intellectual discipline – in part because making moral judgments and decisions is not just an intellectual matter. A very 'thin' kind of ethics based on notions such as reciprocity might be seen as a precondition for any kind of social life and so as uncontroversial, however.
• Is there – as Karl Kraus thought – a deep and intimate link between morality and how we use language? (I don't think so. Not in the way Kraus saw the matter, anyway.)
• What is the cause (and significance?) of the disappearance of the subjunctive and, more generally, of formal and literary modes of speaking and writing?
• What is the source and significance of that strange sense of dread and guilt which some people feel deeply and others don't feel at all? This is one of those many topics which you could approach via psychology or historically. The Etruscans were said to be particularly prone to such feelings, and certain strands of Christian thinking were driven by this sort of thing. There was, I recall, an obscure member of the Vienna Circle who wrote something on this topic (taking a psychological approach). Must look him up.
• The Idealist and Romantic notion of the spirit or genius of a language or a people generally makes far too much of linguistic and cultural groupings, imputing to them not only a life of their own but also a destiny to fulfil, a totally implausible – and very dangerous – idea which is still being energetically propagated today. Culture is crucially important, but clearcut cultural and linguistic boundaries between languages and cultures simply don't exist.
• In fact, the very notion of a (natural) language is problematic. Certainly it represents an abstraction from empirical reality. (Chomsky believes that there are only (overlapping) idiolects.)
• The early-20th century fad for constructed international languages: what was driving it? How did this movement – or competing set of movements – relate to other international movements of the time, like socialism for instance?
• The nature of mathematics. Can mathematics be fitted into a (broadly) empiricist epistemology and/or a naturalistic worldview?