Strained layers will improve one or the other's mobility
but degrade its opposite.
LDMOS structures have better Rds(on)*BV product
than thicker oxides or stacked FETs but need special
process engineered.
Silicided S/D and reduced contact-gate spacing
improves extrinsic resistance. LDD structures that
are used to improve leakage / BV tend to also be
applied to the source where they do no good (Rs
is always worse than Rd except for current mirror
match and drain linearity - degeneration). But the
LDD is often the only thing that hooks the source
/ drain up to the channel, S/D are stood off by the
spacer.
Your options may be mask accessible or may require
process fiddling. What are your tools?