He will be another Seeman but much more successful in splitting anti-DMK votes in favour of DMK. 

1.He genuinely hates DMK and ADMK to the extent. So he will split the anti-DMK votes.

2. DMK can easily brand him as Sanghi given his past membership. Internet is rife with videos of him talking against Christian conversion and other trivial religious issues that only a bigot will be concerned with.  So he can never win popular vote with his Sangi brand, which is a good thing.

3. 50% ( based on my convo )of Joseph's voters are people who fantasize Joseph either as their biological father or husband - they took his , " Ungalalil oruvan naan" rhetoric seriously. These are his die hard fans whom I call as Josephians. 

Rest 50% were genuinely misled by Josephians or simply voted him in his favour, out of boredom. These voters are mostly swing voters with no clear preference for any party. Annamalai can split their votes and spoil the game for both Joseph as well as DMK 

4. He can very well steal the votes of BJP. Most BJP voters in TN, I believe, support them because they genuinely believe DMK is an anti-Hindu party.  Annamalai can split that vote too.

It is almost clear ideologically he will pose himself as anti-DMK - culturally centrist, unionist in policies ( pro - central govt.), economically little right to the DMK but more or less same as DMK. This is the voter 

What is still unanswered is whether he will speak against Joseph and register in the mind of the people that he is a viable option?

During ADMK rule in 2011-16, Seeman was speaking against DMK instead, for their supposed betrayal of Eelam cause. If Annamalai does the same and keeps mum against Joseph and targets DMK instead ,people who voted for Joseph may not move to his camp in droves. He may fail to gain swing votes as well.

It is good for DMK if he floats a separate party