There's a wide-spread consensus that the official Imperial Fists rules are... not great. While there is an old thread which is arguably better, it's not really up-to-date anymore:
So I would like to improve on the fists.
The first thing that has to go is +10 WP. No more IF psyker lulz. In the light of Apocalypse's Primarch chart, +5 T & +5 WP seems more justified.
The second most important thing would be Solo & Squad Mode, as that impacts which remaining traits must be modeled via skills and talents. Given the new chapter rules in 40K 6E, Bolter Drill & Siege Masters, this has to reflect in Squad Modes. Since Siege-Mastery is more fundamental to the IF and Bolter Drill seems to be more Squad Mode-based. So the basic ideas for those abilities fom RoB could remain - just the execution improved.
And here's where it starts:
A. How to model Siege Master instead? I was thinking Evaluate-based and with AP addition/subtraction instead of multiplication.
B. The Defensive Stance cannot remain as-is. It's useless. Nobody will sacrifice Cohesion for that, much less 3 points. I agree that a willpower- and tenacity-based mode would be right. But fearless? For the Deathwatch RPG, this is, by itself, meh. One would need to cheapen the power and tack on something more useful in siege warfare... like shifting position and the effect area. Shifting position is important in siege warfare. Sadly, this has already been covered by Regroup.
C. Tactical Bolter Drill should give a bonus that can be stacked with Bolter Drill talent... like one bonus DoS if the attack hits.
The IF's cheap access to Tactics is ridiculous. It makes the Ultras look like chumps. Eye of Vengeance is supposed to help with siege warfare but it only makes them be good snipers.
One of the main problem is that being siege warfare experts isn't easy to model.