Although Splunk does have a decent capability to transform data, it is FAR from an ETL tool, so IMHO, it would be a poor choice for this right now. I say "right now" because soon (possibly this year in version 8.0 which we all expect to be released at .conf 2019 in a few months), a significantly rearchitected Splunk will be released which should give developers the ability to add code to any portion of the input/index pipeline. Whenever this evolution is released (if it ever is), you will be able to do what you would need to do to transform your data on the way in. Until then, Splunk is not flexible enough to transform data in the way that you need.
If you WERE to select Splunk for this project and force this "uniform" data format mandate upon it, I would tell you to use HEC and select JSON as the output format. This method allows you to send multiple structured formats into Splunk where it will convert it into JSON for indexing: http://dev.splunk.com/view/event-collector/SP-CAAAE6P
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