I myself prefer ties between 21 & 31 to be loose and flexible.
Given DC's track record over the past 25 years, we would likely see:
1) 31st C Tim intro'd into an important, keystone event in Legion mythos; the next 'Great Darkness'-level yarn in 2010 that makes Legion a top seller in the DCU again.
2) TH becomes a reoccurring pivotal figure in the aftermath of said epic. Even without appearing very often, he is woven into the fabric of much of the ongoing plots involving Mordru, Mysa and any other semi-magical LSH character (2011-2015).
3) 21st C Tim killed off in 2016.
4) DC forces Legion to retcon 31st C Tim away, hurting Legion stability and accessibility to readers left scratching their heads (late 2016-7).
5) Legion creators of 2016-2020 spend lots of extra effort handling the unforeseen side effects and editorial directives of #3/4, with the end result leaving LSH even more convoluted than ever before.
6) Tim Hunter is resurrected in the overhyped mini, The Books of Tedium in 2022.
7) Just after LSH continuity has re-settled and adjusted to the absence of TH, 31st C Tim H is reintro'd (2024).
8) In 2025, 21st C Tim Hunter is made half-demon.
9) TPTB make 31st C Tim H be retconed to have always been part-demon.
...and the dance begins again...
I favor as much separation between the two eras as possible, and temporary inconsistencies ignored. I don't trust DC to manage it well enough, based upon their track record.