differentially expressed intron analysis?
Typical tools to use for this would be the DEXseq packages (as done here) or rMATS, which will calculate intron retention rates.

I would generally suggest not summing intronic counts by gene and using that in DESeq2 (I assume that's what you meant by DESeq, since there's no reason to use the original DESeq for anything). What you'd be getting if you did this is less expressed introns and more sample-specific bias in enriching for mature versus pre-mRNA.