For advice on Inner Sphere Omnis I'd go with make them faster. The problem with the canon Omnis is that most of them are heavyweights and slow. One of the strongest things an Omnimech can be is a battle armor carrier and the Clans that exploit this do it well. Since they are more expensive slugging it out with a heavier mech is just wasteful when you could field something like a Strider or Preta, drop off a bunch of heavy armor and support them in their mission.
Leave the slugging it out to cheaper SFE Battlemechs, and flanking to the XL equipped fast cavalry units. They may not be as fast to repair in the field but they are more easily replaced.
Part of the probelm in Canon, is that IS tech tends to weigh more, so to get the same flexibility of function, you have to go with heavier chassis, which in turn angles toward heavier weight classes to get the same (or slightly inferior, but not catastrophically) firepower or payload.
when you look at the early (3055) omnis turned out by Inner Sphere producers, most tend to be on the heavier side for a reason-to have enough combined carry weight and space to be worth (more than) a standard 'mech.
Clan Nova/Blackhawk: 50 tons
Blackhawk-KU: 60 tons to get the same movement profile and inferior range and damage in the prime configuration. (Just as a 'for example' example).
Now, if you're 'clean sheeting' it, and NOT trying to copy what the Clans already fielded against you, there are a lot more options for layout and weapons load-out, for any given chassis weight and design style...but you're likely to gravitate toward the Heavy end of the spectrum if you want it to be viable in more than a single specialist config. That's just the limits of IS tech.