aluminum rods vs. titanium rods: fatigue life

If you want to start down the rabbit hole, search 'SN curves' and do some reading. You'll be lucky to find good graphs of certain materials and it will become obvious pretty quickly why most rods aren't aluminum.
Forged steel is tough to beat. I'm sure there could be some tool steels or high-strength steels which could be used (many high-strength steels lack fatigue life - it's a trade off for this super high strengths, but not always).
If you can increase the strength, obviously you can reduce the mass which can further reduce the load which reduces the mass etc. This level of optimization can be quite an engineering challenge, and inputs to calculate an acceptable performance window are not always readily available. Lots of minor stuff also becomes a concern - stress risers, imperfections, finish methods etc.
Quite a few high output OEM engines are using powdered metal rods these days, but tooling and testing can eat up $millions in development. Still, papers on those things might help size some of the loading to do some initial investigation.