I have been drawn strongly to the goddess Diana and (not as strongly) to Cernunnos. Would worshipping deities from different pantheons cause issues, or is it not a big deal? Thanks

As with many things: It depends. If you're not a hard polytheist, there may be no issues at all; it's less of a problem to reconcile that sort of thing if you see the deities in question as archetypes or facets of a larger being or something, I think.
Beyond that, I think part of the question is
how you're doing this. If you're trying to worship them together, and do things that are appropriate for both of them at the same time, that will require a lot more thought than if you want to worship each of them separately. Coming at this from a hard-polytheist standpoint, I'd also consider how the deities in question might feel about each other. Even within a single pantheon you'll find some deities that don't get along with one another and might not like sharing devotees.
Finally, there's also the consideration of whether the particular religion/tradition/path you're working within has anything to say about it. There's not some universal rule saying you can't mix pantheons, but some traditions might have more strict definitions of how things should be done.