There are no constraints regarding the fronting of adverbs in German. What you need to bear in mind is that the verb must succeed the adverb. To be more precise, there are some adverbs that tend to occur in first place, such as "vielleicht", and others that can also figure in another slot, such as "leider" ('unfortunately), as in "leider bist du zu spät" ('unfortunately you are too late") or "du bist leider zu spät". Both "vielleicht" and "leider" are so called "sentence adverbs" that express the speaker's attitude towards the described event. In contrast, if you are referring to adverbs altogether, a more fine-grained distinction and categorisation would be required. For example, adverbs of manner such as "schnell" ('quickly') would only occupy a sentence-initial slot for purposes of emphasis: "schnell rannte er weg" (literally 'quickly he ran away'). In all these cases, however, the adverb must be followed by the verb.