What interests you most about autism? I suggest you read various topics and find which one catches your fascination, and do something on that topic.
Given that you're focusing on early childhood, you may want to study early diagnosis and/or early intervention as those are big issues for that age group.
One topic I'd be really interested in seeing studied is what impact ABA therapy has on sensory sensitivities, given that ABA trains changes in behaviors motivated by sensory issues (such as avoidance of eye contact) without addressing the underlying sensory sensitivity. If a child who avoids eye contact due to hypersensitivity starts making eye contact, has their sensitivity gone away, or have they just learnt to hide their reaction?
Another topic might be parent-child relationships. Studies show that autistic kids have normal attachment behaviors, despite lower rates of non-attachment social behavior. However, parent accounts often suggest that parents think their autistic children are not attached to them. Do parents tend to underestimate the level of attachment the child would show in the Strange Situation? Does underestimating attachment predict those parents at worst risk of depression and parenting stress? Would teaching parents to recognize their child's attachment behaviors help the parent relate better to the child?
Another intervention study idea - ABA therapy has been extensively studied, but many other treatments, such as Floortime/RDI, have only limited research. One particular question is whether different kinds of kids benefit from different therapies. For example, Elizabeth Newson has described a subtype of kids who react very negatively to direct commands. Are there some children who respond better to Floortime because it doesn't involve giving the child commands?