I highly doubt any real "dedicated" melee-addition roster unit to the IG early-game would be quite a beneficial and good addition to the IG in general. The natural aspect of common-build of guardsmen are still applied naturally, and IG does have good "melee"-counteraction measures by purely distruptive-means and all that jibberish. Their anti-melee is quite broad-but subtle in nature really. It's not strictly a counter-productive investment requirement, and is more forced on agression/distruption in an essence.
Sentinel, as it stands. Has quite good means to counter at least 1/2 melee-blobs. If the ability missfires, of course- it'll take it's toll. But there's no excuse to perhaps admit a mistake on it's useage that can misslead to a unappropiate engagement resulting in a faulty map control loss. I've seen this so many times, sometimes missused, and sometimes critical uses that pushes off key-centric units.
Even existing unit template-builds needs to be carefully thought of when implementing generally any new-units to the existing factions as well. For an example, that's why the Sternguard/Vanguard veterans were approached, since naturally the SM economy wouldn't re fully operational and functional to work with a unit of it's design and misc. to make up for a wide-variety of an build-order. Hench they got tied to the Tac/ASMs, since it was the only real way to counter-balance the fundemental desgin of the unit alongside with the race economical structure.
I fear the implementations suggestions as well, will onfortunately fall flat in this regard. However, as propositions are made, the intentions are good, and the thought is kind & well in theory. The practical play can be quite counter-productive as well. If we were to imagine a standard-issued GM(default) / sent, Guardsmen, builds - followed with certain propositions. You would have quite the flexible army capable of tackling with the most variant types, eventho you don't really pack alot of bucket into the units to be fully operational, compared to other races.
- This is also why IG was also one of the heaviest-fast techers in the game (from retail) since they lacked the backbone requirement to pool in resource-fundings into their early-game, so that's why the fast-chimera had also so much of a good-pay off. Since the economical structure and meta of the IG simply just rewarded fast-tech into T2 so heavily. Chaos also shared a similiar trait of this, prior to the latest-Chaos Heretic AC pricing going to 95/30 in retail. - Coupled with the faster-build time on the Bloodcrusher. The pricing adjustments made so the AC provided with enough agression to keep up with the momentum of the game, and reward with a fast-tech T2-crusher, since the general counter-measures would require alot more time, and the opposing factions require more economical pooling into their early-game.
What I'm trying to say is. IG doesn't really struggle with "melee" units, I honest to god, don't know who does really as an IG player. They really do have a good asset available to them, IG as a ground-up race is just not designed to have a "real" melee-unit apart from the Ogryns. Catachans are much more riskier to put into the fray, since you're more prone to losing models, and losing models with Catachan, in terms of quantity IS never a good thing, so people are more generally careful with them then in their retail-state, where it doesn't have that quite much of an impact on them. The Catachans are generally meant to be supportive units with their abilities. They are able to grant you excellent pressure play with Guardsmen/Sent play through smoke bombs (If anyone has missed it. It REDUCES INCOMING ranged damage by 80%, the OUTGOING was removed) - (Which btw, is getting changed in the next patch, lulz)
The pressure play-potentiality is downright excellent from Catachans. But they need to be utilized in this fashion to get the great pay-off. Or, otherwise resort to more conventional play, by placing booby traps, since they're not that taxing on your general upkeep anymore, so it's more beneficial to utilize it more often then not.
If it's a matter of "I don't want to get 4 squads in Tier 1 to do well" - then I'd suggest with another strategical approach to the matter. I advice to go with 2x power-nodes, gen-up both farms if possible, if not go with 3x1/2 farm build. If possible, even node in a frontier location to keep up the pressure, since power nodes generate you an extra-net of extra delay-time + power income as long as it is up. It's a matter of making the most use of it if you want to. But the fast-power node on the frontlines are great ways of stalling your opponent, which can be critical. It's also more useable with lower-sized armies. So it's more of a preference in terms of playstyle, which I think is more likely the case of inefficiency sometimes in some matchups and etc;
If anything, IG probably deserve some general cost-effiency tweaks, but that's more pricing adjustments more then anything. The IG economy can sometimes go abit wonky in their mid-game, but if they go for a fast-tech T3 from a good strong performance in T1, it can have great pay-off in late-game. It's more when IG decides to go with some T2 investments, they're more likely to not have the option to go T3 later-on, with an en-masse combination of T1/T2 army, since IG mid-game is quite power-dependant, coupled with some attrition-based losses that are more or less unavoidable.
For example,Tyrannids did not need DoM but they still can be beat even when they have it.
I'm sorry, what?