8th edition Reserve Rules mean that "null" armies are effectively illegal for matched play. While this means that certain armies (the dreaded Warp Spider Army of Doom) are just illegal, this also hoses anyone that wants to do a "from the skies" or any "null" army, be it Farsight Tau, Elysians, Black Legion Spearheads, etc. Add the fact that you need to have a special rule to be explicitly allowed to reserve on top of this.
The problem with this solution is that rather than creating core rules to develop a viable counter, it simply takes away an option altogether. ("Dear Games Workshop, Please nerf Paper. Scissors are fine. Signed, Rock."). It's this same mentality that leads to debacles like the MC02 Wargame Challenge, where nobody wins the game in the long run.
Given that 7th edition also has the issue that assault is either "go big or go home" (meaning either a null-melee army like Genestealer Cults/Death Guard Raptor Talons, or a Cabalstar/Barkbarkstar/etc), why not kill several birds with one stone?
Let's merge Interceptor and Overwatch into a single mechanic:
Any unit in 40k can Overwatch unless specifically noted otherwise. A unit that forfeits shooting, running/flat-outing, or charging may enter Overwatch; Overwatch lasts until the start of your next turn if it's otherwise unused.
During the opponent's turn (meaning Overwatch attacks cannot trigger counter-Overwatch attacks), when an enemy unit wishes to shoot, charge, or manifest a Malediction/Witchfire, the Overwatching unit may immediately interrupt the opponent's game turn to shoot that, provided the enemy unit either:
A) Is within 12" of the Overwatching unit.
B) Is targeting or would otherwise affect either the Overwatching unit, or a friendly unit within 6" of the Overwatching Unit. (This means even non-targeted powers like Beams and Novas would trigger Overwatch.
Overwatch is resolved at -1 BS to a minimum of BS 1, and is based on the movement you took in your previous turn. Flamers are treated as normal, or they have the option to perform Wall of Death against units charging the Overwatching Unit, which is modified as follows:
-Wall of Death does D6 hits, but may only remove models within 12" of the flamer.
Any abilities dealing with Interceptor, improved Overwatch BS, Grim Resolve, Supporting Fire, etc, are replaced with the following USR:
Sentinel: Ignore the -1 BS penalty to Overwatch.
I feel the system is simple enough (more streamlined than 2e's "overwatch past the blind grenade" stalemates that could occur) and promotes combined arms support. Plus, there are some cute tricks that you can still do with it that could be promoted, like conga-lining a Guard blob up the field, with Wyverns on overwatch to protect the advance as an abstraction of rolling barrage tactics 101 (Note: This could technically also be done with a Barkbarkstar, but that's another issue for another "alternative rule proposal").
Add in an extra footnote that says that Warp Spiders cannot Flickerjump during the Eldar player's turn to round it off.