Wait for updates, I guess? While not a confirmed or announced thing, it's commonly supposed that the rules being free online, or cheap in the compendiums, is to allow for regular updates without leaving players feeling like they wasted a lot of money on outdated rules. It would not surprise me at all to see new compendiums with revised faction traits, items, and spell lists published next year.
For now, though? Fortunately, our default traits and items in the general's handbook are pretty nice overall. And our only published battletome so far, the fleash-eaters, aren't especially magic-dependent, so the rule of one doesn't hit them too hard, even with the lack of a spell lore of their own.
As for the rest of the faction - yeah, rules of one and the summoning restriction are heavy blows. I haven't looked closely at the points values of other factions to see if the effect of those restrictions have been taken into account. Nagash in particular seems a little overpriced once you consider those restrictions, but then again the impact of the casting & summoning nerfs seems to have been taken into account with Arkhan's points cost, which is fully 100 points cheaper than the next cheapest Mortarch, so maybe it will be ok.
For those who want to lean heavy on magic - it is part of the whole undead theme after all, I recommend acquiring or converting some of the tomb kings statue units - sphinxes, ushabti, necropolis knights, and stalkers. All of those units have powerful healing spells, which shouldn't suffer from the summoning nerf. Necroknights and Stalkers can even be raised above starting strength by them, and sphinxes and necroknights are very respectable units to begin with. That should get you some extra useful spells to cast to get around the rule of one which don't require you to set aside points for summoning.
Tomb Kings are hard to come by these days, some conversion/counts-as ideas:
FW had some concept art for a tomb kings giant animated beetle thing that never materialized, but Alarielles beetle might make a solid foundation for a giant scarab conversion that could reasonably be run as one of the sphinxes. You'd need to find a use for alarielle (trade to a chaos player looking for daemon prince conversion fodder?), and it would require a lot of work to remove all the plants and vines, but it's an idea.
Buy and assemble some hexwraiths, and use the leftover black knight arms and upper torsos to convert undead riders for demigriph knights, and paint the griphs as ghosts or animate statues. Should make for a respectable vamp count themed necroknight stand-in. I actually have necroknight models, and I'm tempted to do this, as the snake surfers are a bit silly....
Not GW store legal, but Mantic is putting out a not-tomb-kings range in the next month or two, with some halfway-decent not-ushabti, bow option included.
