I was a casual player, although I wouldn't fall into that category anymore for FAF given I play a fair bit of TMM, but going back to when I was more of a casual player the thing I would play the most would be the SC and FA campaign, and before I even considered trying to play custom games I'd play a number of vs AI games (I had no idea custom AI existed so these would have been vs the standard AI).
Some if the things I think FAF could improve to appeal more to casual players who play PvE would be:
- Improved interface/client for the campaign side of the game - it's not obvious/intuitive that you can play solo games through the SC campaign (but with FAF balance and UI), or that there are a bunch of custom missions/campaigns, or the order to play these in.
- Improved visibility/prominence of AIs, including AI selection - many players won't know custom AI exist and will assume there's only the default AI available. Even if you know custom AI exist, unless you happen to go looking for it on the forums you won't easily find them, or know how difficult each of them is.
- Some sort of 'challenge mode' and/or tutorial - e.g. C&C Zero Hour had the general challenge which I found great fun and kept me going back to the game time and time again.
Of these the last is probably not worth considering since it'd require lots of time and such ideas have already been suggested in the past. The first two probably wouldn't require as much work, but would still need someone with experience of the client and a fair bit of time.
In terms of your suggestion for easy to very hard groupings for the AI, I think this could work as a good idea; giving a rating number to the AI would be an alternative (and is discussed in the AI matchmaker thread), but they both have the same aim, and for someone new to FAF easy/medium/hard is probably more understandable than 200 / 400 / 600. However, I think this should be coupled with a complete rework of the UI relating to AI in the lobby.
I'm not sure quite what the UI would look like/the best approach, but ideally it would be something that would allow you to easily select an AI of the appropriate challenge, while also making you aware of all the possible AI.
For example, one approach might be a single 'Add AI' option. You select it, and you then get a box/grid that lists all available AI (i.e. including AI for which you dont have the mod for). Selecting an AI that you dont have the mod for yet will download that mod automatically. This at a stroke would massively increase awareness of the different AI that FAF has for newer players.
A box/grid/some other solution would also give potential for AI to be grouped/sorted. E.g. you could have them grouped by difficulty; you could us a tag system; you could have them grouped by type of AI (rush; tech; adaptive).
Meanwhile with the AiX modifier, instead of needing to select the AiX version of the AI (which doubles the list of AI available), and then know to go into game settings to change it from the default (now of 1.5), if you select an AI then there could be a drop-down box where it shows the AI/player name in the lobby that is at 1.0, and allows you to select any AiX value (for that AI), including values less than 1.0 (to cater for players that want an even easier time).
If the achievement system was working, this could also be used to encourage people to explore some of the FAF content (e.g. an achievement for beating an AiX 1.5 Adaptive AI, or for completing a custom campaign).
Another potentially simpler approach (although it would have less benefit) would be adding a section in the mods part of the client just for AI mods (i.e. the same way things are grouped by highest rated, most popular, etc., if there's a grouping for AI mods)