So, I've been reading my new book, Introduction to Typology: The Unity and Diversity of Language. I've just encountered something I like a lot, and plan to use it in Caia. Let me attempt to give a very brief amount of background and explanation.
Not all languages have tense. And some languages on both sides have something called aspect. Aspect indicates some facet of time about a verb in ways that are more complicated than tense. For example, some languages conjugate the same verb (without a helper verb, which English uses) differently depending on whether the subject is starting to do something (e.g. "began to eat"), finishing something (e.g. "finished eating"), etc.
One very common class of aspect (although usually this aspect in intrinsic to the verb, and does not have multiple conjugations for the same verb) is the distinction between state, achievement, activity, and accomplishment. The state aspect indicates that the subject is maintaining some preexisting state. The achievement aspect indicates a change in state. The activity aspect indicates some act that is self-contained and does not include a change in state. Finally, the accomplishment aspect indicates that the subject has caused an action with regard to some other object.
Some examples in English, using the verb 'break' (note that English cannot perfectly distinguish these four aspects for individual verbs, so try not to get confused by the ambiguity). The state aspect: "the window is broken" (this is a little ambiguous; specifically what that says is that the window was broken before the present time, and continues to be broken). The achievement aspect: "the window broke" (also "the window was broken", but that could also indicate the state aspect). The activity aspect: "I ran to the window". The accomplishment aspect: "the ball broke the window".
As this very elegantly accounts for the active and passive voices, as well as both transitive and intransitive verbs, I'm gonna definitely use those as some of the mandatory aspects of Caia (some aspects - the most useful and common ones - and possibly tense are included in the basic conjugation of the verb, while other aspects are formed, as in English, either through phrases or extra words such as adverbs). Though I may combine the achievement and activity aspects, as I'm hard-pressed to find a verb for which activity and achievement are not mutually exclusive (note how 'break', by its very definition, could not display the activity aspect, so I had to use a different verb). And for anyone who's curious: the jury is still out on whether Caia will use tense.